[Phase 2.1] Synthesis workflow definition (#12)

## Changes
- `workflow/synthesis.go` — 5-stage synthesis pipeline as Temporal workflow
- `workflow/synthesis_test.go` — 5 tests using Temporal test framework

## Pipeline Stages
1. **ChunkAndEmbed** — chunk text + generate embeddings
2. **ExtractEntities** — LLM entity extraction with reflection
3. **ExtractFacts** — pattern + LLM fact extraction
4. **DetectContradictions** — pre-filter + LLM verification
5. **PersistSynthesis** — save all results to DB

workflow/synthesis.go

@@ -0,0 +1,125 @@
+package workflow
+
+import (
+	"fmt"
+	"time"
+
+	"go.temporal.io/sdk/temporal"
+	"go.temporal.io/sdk/workflow"
+	"github.com/rockliang/poimen/workflows/pkg/types"
+)
+
+// Re-export shared types from pkg/types for backward compatibility
+type SynthesisInput = types.SynthesisInput
+type SynthesisResult = types.SynthesisResult
+type ExtractedEntity = types.ExtractedEntity
+type ExtractedFact = types.ExtractedFact
+type ContradictionResult = types.ContradictionResult
+type PersistInput = types.PersistInput
+
+var synthesisActivityOptions = workflow.ActivityOptions{
+	StartToCloseTimeout: 60 * time.Second,
+	RetryPolicy: &temporal.RetryPolicy{
+		InitialInterval:    time.Second,
+		BackoffCoefficient: 2.0,
+		MaximumInterval:    30 * time.Second,
+		MaximumAttempts:    3,
+	},
+}
+
+// SynthesisWorkflow orchestrates the 4-stage memory synthesis pipeline.
+//
+// Stage 1: Chunk + embed text
+// Stage 2: Extract entities (LLM + reflection)
+// Stage 3: Extract facts (pattern + LLM)
+// Stage 4: Detect contradictions (pre-filter + LLM)
+//
+// Each stage is an activity with independent retry policy.
+func SynthesisWorkflow(ctx workflow.Context, input SynthesisInput) (*SynthesisResult, error) {
+	logger := workflow.GetLogger(ctx)
+	startTime := workflow.Now(ctx)
+
+	logger.Info("synthesis started",
+		"project", input.Project,
+		"source", input.Source,
+		"kind", input.Kind,
+	)
+
+	actCtx := workflow.WithActivityOptions(ctx, synthesisActivityOptions)
+
+	// Stage 1: Chunk + Embed
+	var chunkID string
+	err := workflow.ExecuteActivity(actCtx, "ChunkAndEmbedActivity", input).Get(ctx, &chunkID)
+	if err != nil {
+		return nil, fmt.Errorf("stage 1 chunk+embed: %w", err)
+	}
+	logger.Info("stage 1 complete", "chunk_id", chunkID)
+
+	// Stage 2: Entity Extraction
+	var entities []ExtractedEntity
+	err = workflow.ExecuteActivity(actCtx, "ExtractEntitiesActivity", chunkID, input.Text).Get(ctx, &entities)
+	if err != nil {
+		return nil, fmt.Errorf("stage 2 entity extraction: %w", err)
+	}
+	logger.Info("stage 2 complete", "entities", len(entities))
+
+	// Stage 3: Fact Extraction
+	var facts []ExtractedFact
+	err = workflow.ExecuteActivity(actCtx, "ExtractFactsActivity", chunkID, input.Text, entities).Get(ctx, &facts)
+	if err != nil {
+		return nil, fmt.Errorf("stage 3 fact extraction: %w", err)
+	}
+	logger.Info("stage 3 complete", "facts", len(facts))
+
+	// Stage 4: Contradiction Detection
+	var contradictions []ContradictionResult
+	err = workflow.ExecuteActivity(actCtx, "DetectContradictionsActivity", input.Project, facts).Get(ctx, &contradictions)
+	if err != nil {
+		return nil, fmt.Errorf("stage 4 contradiction detection: %w", err)
+	}
+
+	reviewQueued := 0
+	for _, c := range contradictions {
+		if c.QueuedReview {
+			reviewQueued++
+		}
+	}
+	logger.Info("stage 4 complete", "contradictions", len(contradictions), "review_queued", reviewQueued)
+
+	// Stage 5: Persist results
+	persistInput := PersistInput{
+		ChunkID:        chunkID,
+		Project:        input.Project,
+		Source:         input.Source,
+		Kind:           input.Kind,
+		Entities:       entities,
+		Facts:          facts,
+		Contradictions: contradictions,
+	}
+	err = workflow.ExecuteActivity(actCtx, "PersistSynthesisActivity", persistInput).Get(ctx, nil)
+	if err != nil {
+		return nil, fmt.Errorf("stage 5 persist: %w", err)
+	}
+
+	duration := workflow.Now(ctx).Sub(startTime)
+	result := &SynthesisResult{
+		ChunkID:           chunkID,
+		EntitiesExtracted: len(entities),
+		FactsExtracted:    len(facts),
+		Contradictions:    len(contradictions),
+		ReviewQueued:      reviewQueued,
+		Entities:          entities,
+		Facts:             facts,
+		Duration:          duration,
+	}
+
+	logger.Info("synthesis complete",
+		"chunk_id", chunkID,
+		"entities", len(entities),
+		"facts", len(facts),
+		"contradictions", len(contradictions),
+		"duration", duration,
+	)
+
+	return result, nil
+}