refactor: rename action→activity, statemachine→workflow, remove HTTP API layer

- action/ → activity/ (Temporal activities)
- statemachine/ → workflow/ (Temporal workflows)
- Removed internal/api/ and cmd/server/ (api-gw handles HTTP, Temporal is the API)
- Created pkg/types/types.go as single source of truth for all shared types
- Extracted CallRoleLLM helper (DRY: implementer/planner/judge shared pattern)
- Fixed circular import: workflow_graph_query uses string activity names
- Fixed logger.logf → logger.Info/Warn (method didn't exist)
- Fixed routing types: added Branches, Activity, BackoffSeconds, TaskActivity
- Fixed db.Canvas.Name, db.Client→DB, GetWorkflow→FetchWorkflow
- Removed unused imports
- All tests pass, build clean, vet clean

activity/implementer.go

@@ -0,0 +1,41 @@
+package activity
+
+import (
+	"context"
+
+	"github.com/rockliang/poimen/workflows/pkg/types"
+	"go.temporal.io/sdk/activity"
+)
+
+type ImplementerInput struct {
+	Config       types.OrchestratorConfig
+	TaskID       string
+	WorktreePath string
+	Lessons      string
+}
+
+type ImplementerOutput struct {
+	Success bool
+	Changes string
+}
+
+func ImplementerActivity(ctx context.Context, in ImplementerInput) (ImplementerOutput, error) {
+	activity.RecordHeartbeat(ctx, "starting implementer for "+in.TaskID)
+
+	vars := map[string]any{
+		"SystemPrompt": in.Config.SystemPrompt,
+		"Task":         in.TaskID,
+		"WorktreePath": in.WorktreePath,
+	}
+	if in.Lessons != "" {
+		vars["Lessons"] = in.Lessons
+	}
+
+	response, err := CallRoleLLM(ctx, in.Config, "implementer", vars)
+	if err != nil {
+		return ImplementerOutput{}, err
+	}
+
+	activity.RecordHeartbeat(ctx, "implementer completed for "+in.TaskID)
+	return ImplementerOutput{Success: true, Changes: response}, nil
+}