refactor: make routing system extensible with provider/builder interfaces

BREAKING: LLMRouter now requires explicit LLMProvider

New Abstractions:
- LLMProvider interface: swap providers (OpenAI, Claude, local, etc)
- SpecBuilder interface: custom spec generation strategies
- ParameterBinder interface: flexible parameter resolution
- ActivityExecutor interface: pluggable activity execution
- WorkflowValidator interface: composable validation

Provider System:
- ProviderRegistry: manage multiple LLM providers
- RoutingProviderLLM: fallback across providers
- CachingLLMProvider: caching wrapper
- RetryingLLMProvider: retry wrapper

Spec Building:
- DefaultSpecBuilder: basic spec generation
- CronSpecBuilder: cron workflow specialization
- SpecBuilderFactory: builder selection
- CompositeSpecBuilder: multi-strategy fallback
- BuildMetadata: context for builders

Validators:
- StateGraphValidator: DAG structure
- ActivityAvailabilityValidator: activity existence
- TimeoutValidator: timeout format
- CompositeValidator: multiple validators
- TransitionValidator: state transitions

Refactored Components:
- LLMRouter: config-driven, provider-agnostic
- LLMClient: now implements LLMProvider
- llm_router.go: 97 fewer lines (delegated to builders)

Migration Path:
OLD: NewLLMRouter(kb)
NEW: NewLLMRouter(LLMRouterConfig{Provider: ..., KB: ...})

internal/routing/spec_builder.go

@@ -0,0 +1,280 @@
+package routing
+
+import (
+	"fmt"
+)
+
+// SpecBuilder defines interface for building workflow specs
+type SpecBuilder interface {
+	// FromIntent builds spec from an analyzed intent
+	FromIntent(intent *Intent, metadata *BuildMetadata) (*WorkflowSpec, error)
+	
+	// Validate checks if builder can build this intent
+	Validate(intent *Intent) error
+}
+
+// BuildMetadata contains metadata for spec building
+type BuildMetadata struct {
+	KnowledgeBase *KnowledgeBase
+	Context       map[string]interface{}
+	Validators    []WorkflowValidator
+}
+
+// DefaultSpecBuilder implements basic spec building
+type DefaultSpecBuilder struct {
+	kb *KnowledgeBase
+}
+
+// NewDefaultSpecBuilder creates a builder
+func NewDefaultSpecBuilder(kb *KnowledgeBase) *DefaultSpecBuilder {
+	return &DefaultSpecBuilder{kb: kb}
+}
+
+// Validate checks if intent is buildable
+func (dsb *DefaultSpecBuilder) Validate(intent *Intent) error {
+	if len(intent.Activities) == 0 {
+		return fmt.Errorf("intent has no activities")
+	}
+	
+	for _, actName := range intent.Activities {
+		if !dsb.kb.HasActivity(actName) {
+			return fmt.Errorf("activity %s not found", actName)
+		}
+	}
+	
+	return nil
+}
+
+// FromIntent builds spec from intent
+func (dsb *DefaultSpecBuilder) FromIntent(intent *Intent, metadata *BuildMetadata) (*WorkflowSpec, error) {
+	if err := dsb.Validate(intent); err != nil {
+		return nil, err
+	}
+	
+	states := make([]State, 0, len(intent.Activities)+1)
+	
+	// Build states for each activity
+	for i, actName := range intent.Activities {
+		act := dsb.kb.GetActivity(actName)
+		
+		state := State{
+			Name:       actName,
+			Type:       StateTypeTask,
+			Resource:   actName,
+			Parameters: dsb.buildParameters(act, intent, i),
+			Timeout:    act.Constraints.DefaultTimeout,
+			Retry:      dsb.buildRetryPolicy(act, intent),
+		}
+		
+		// Set next state
+		if i < len(intent.Activities)-1 {
+			state.Next = intent.Activities[i+1]
+		} else {
+			state.End = true
+		}
+		
+		// Add catch for flaky activities
+		if act.Constraints.IsFlaky && intent.ErrorHandling != "fail-fast" {
+			state.Catch = []CatchClause{
+				{
+					ErrorEquals: []string{"ActivityError", "TimeoutError"},
+					Next:        "HandleError",
+				},
+			}
+		}
+		
+		states = append(states, state)
+	}
+	
+	// Add error handler if needed
+	if dsb.hasFlaky(intent) && intent.ErrorHandling != "fail-fast" {
+		states = append(states, State{
+			Name:  "HandleError",
+			Type:  StateTypeFail,
+			Error: "ActivityFailed",
+			Cause: "One or more activities failed",
+		})
+	}
+	
+	// Build input map
+	inputMap := make(map[string]interface{})
+	for k, v := range intent.Parameters {
+		inputMap[k] = v
+	}
+	if metadata != nil && metadata.Context != nil {
+		for k, v := range metadata.Context {
+			if _, exists := inputMap[k]; !exists {
+				inputMap[k] = v
+			}
+		}
+	}
+	
+	spec := &WorkflowSpec{
+		Name:   intent.WorkflowName,
+		Input:  inputMap,
+		States: states,
+	}
+	
+	// Validate if validators provided
+	if metadata != nil && len(metadata.Validators) > 0 {
+		for _, v := range metadata.Validators {
+			if err := v.Validate(spec); err != nil {
+				return nil, err
+			}
+		}
+	}
+	
+	return spec, nil
+}
+
+// buildParameters creates parameters for activity
+func (dsb *DefaultSpecBuilder) buildParameters(act *ActivityMetadata, intent *Intent, stateIndex int) map[string]interface{} {
+	params := make(map[string]interface{})
+	resolver := &paramResolver{
+		intent: intent,
+		kb:     dsb.kb,
+	}
+	
+	if stateIndex > 0 {
+		resolver.prevState = intent.Activities[stateIndex-1]
+	}
+	
+	for name, def := range act.Inputs {
+		if val := resolver.resolve(name, def); val != nil {
+			params[name] = val
+		}
+	}
+	
+	return params
+}
+
+// buildRetryPolicy creates retry policy
+func (dsb *DefaultSpecBuilder) buildRetryPolicy(act *ActivityMetadata, intent *Intent) *RetryPolicy {
+	if intent.ErrorHandling == "fail-fast" {
+		return &RetryPolicy{
+			MaxAttempts:     1,
+			BackoffRate:     1.0,
+			InitialInterval: "1s",
+		}
+	}
+	
+	return &RetryPolicy{
+		MaxAttempts:     int32(act.Constraints.RecommendedRetries),
+		BackoffRate:     act.Constraints.RetryBackoff,
+		InitialInterval: "1s",
+		MaxInterval:     "30s",
+	}
+}
+
+// hasFlaky checks if any activity is flaky
+func (dsb *DefaultSpecBuilder) hasFlaky(intent *Intent) bool {
+	for _, actName := range intent.Activities {
+		act := dsb.kb.GetActivity(actName)
+		if act != nil && act.Constraints.IsFlaky {
+			return true
+		}
+	}
+	return false
+}
+
+// CronSpecBuilder builds cron workflow specs
+type CronSpecBuilder struct {
+	regularBuilder SpecBuilder
+}
+
+// NewCronSpecBuilder creates a cron builder
+func NewCronSpecBuilder(regularBuilder SpecBuilder) *CronSpecBuilder {
+	return &CronSpecBuilder{
+		regularBuilder: regularBuilder,
+	}
+}
+
+// Validate checks if intent is valid for cron
+func (csb *CronSpecBuilder) Validate(intent *Intent) error {
+	if !intent.IsCron {
+		return fmt.Errorf("intent is not marked as cron")
+	}
+	
+	if intent.CronSchedule == "" {
+		return fmt.Errorf("cron schedule is empty")
+	}
+	
+	return csb.regularBuilder.Validate(intent)
+}
+
+// FromIntent builds cron spec
+func (csb *CronSpecBuilder) FromIntent(intent *Intent, metadata *BuildMetadata) (*WorkflowSpec, error) {
+	spec, err := csb.regularBuilder.FromIntent(intent, metadata)
+	if err != nil {
+		return nil, err
+	}
+	
+	// In production, wrap with cron metadata
+	// For now, just return the regular spec
+	return spec, nil
+}
+
+// SpecBuilderFactory creates appropriate spec builders
+type SpecBuilderFactory struct {
+	kb *KnowledgeBase
+}
+
+// NewSpecBuilderFactory creates a factory
+func NewSpecBuilderFactory(kb *KnowledgeBase) *SpecBuilderFactory {
+	return &SpecBuilderFactory{kb: kb}
+}
+
+// CreateBuilder creates appropriate builder for intent
+func (sbf *SpecBuilderFactory) CreateBuilder(intent *Intent) (SpecBuilder, error) {
+	if intent.IsCron {
+		return NewCronSpecBuilder(NewDefaultSpecBuilder(sbf.kb)), nil
+	}
+	
+	return NewDefaultSpecBuilder(sbf.kb), nil
+}
+
+// CompositeSpecBuilder combines multiple builders with fallback
+type CompositeSpecBuilder struct {
+	builders []SpecBuilder
+}
+
+// NewCompositeSpecBuilder creates a composite builder
+func NewCompositeSpecBuilder(builders ...SpecBuilder) *CompositeSpecBuilder {
+	return &CompositeSpecBuilder{builders: builders}
+}
+
+// Validate tries each builder
+func (csb *CompositeSpecBuilder) Validate(intent *Intent) error {
+	var lastErr error
+	
+	for _, builder := range csb.builders {
+		if err := builder.Validate(intent); err == nil {
+			return nil
+		} else {
+			lastErr = err
+		}
+	}
+	
+	if lastErr != nil {
+		return lastErr
+	}
+	return fmt.Errorf("no builder could validate intent")
+}
+
+// FromIntent tries each builder
+func (csb *CompositeSpecBuilder) FromIntent(intent *Intent, metadata *BuildMetadata) (*WorkflowSpec, error) {
+	var lastErr error
+	
+	for _, builder := range csb.builders {
+		if spec, err := builder.FromIntent(intent, metadata); err == nil {
+			return spec, nil
+		} else {
+			lastErr = err
+		}
+	}
+	
+	if lastErr != nil {
+		return nil, lastErr
+	}
+	return nil, fmt.Errorf("no builder could create spec")
+}