feat(synthesis): Add synthesis step for convoy formulas (gt-v5s0j)

Implements the synthesis step that combines leg outputs into a final
deliverable for convoy workflows:

- `gt synthesis start <convoy-id>` - Start synthesis after verifying
  all legs are complete, collecting outputs, and slinging to polecat
- `gt synthesis status <convoy-id>` - Show synthesis readiness and
  leg completion status
- `gt synthesis close <convoy-id>` - Close convoy after synthesis

Key features:
- Collects leg outputs from formula-defined paths (e.g., findings.md)
- Creates synthesis bead with combined context from all leg outputs
- Integrates with formula.toml synthesis configuration
- Provides TriggerSynthesisIfReady() for automated synthesis trigger
- Adds SynthesisFields struct for tracking synthesis state in beads

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/synthesis_test.go

@@ -0,0 +1,94 @@
+package cmd
+
+import (
+	"testing"
+)
+
+func TestExpandOutputPath(t *testing.T) {
+	tests := []struct {
+		name      string
+		directory string
+		pattern   string
+		reviewID  string
+		legID     string
+		want      string
+	}{
+		{
+			name:      "basic expansion",
+			directory: ".reviews/{{review_id}}",
+			pattern:   "{{leg.id}}-findings.md",
+			reviewID:  "abc123",
+			legID:     "security",
+			want:      ".reviews/abc123/security-findings.md",
+		},
+		{
+			name:      "no templates",
+			directory: ".output",
+			pattern:   "results.md",
+			reviewID:  "xyz",
+			legID:     "test",
+			want:      ".output/results.md",
+		},
+		{
+			name:      "complex path",
+			directory: "reviews/{{review_id}}/findings",
+			pattern:   "leg-{{leg.id}}-analysis.md",
+			reviewID:  "pr-123",
+			legID:     "performance",
+			want:      "reviews/pr-123/findings/leg-performance-analysis.md",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := expandOutputPath(tt.directory, tt.pattern, tt.reviewID, tt.legID)
+			if got != tt.want {
+				t.Errorf("expandOutputPath() = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestLegOutput(t *testing.T) {
+	// Test LegOutput struct
+	output := LegOutput{
+		LegID:    "correctness",
+		Title:    "Correctness Review",
+		Status:   "closed",
+		FilePath: "/tmp/findings.md",
+		Content:  "## Findings\n\nNo issues found.",
+		HasFile:  true,
+	}
+
+	if output.LegID != "correctness" {
+		t.Errorf("LegID = %q, want %q", output.LegID, "correctness")
+	}
+
+	if output.Status != "closed" {
+		t.Errorf("Status = %q, want %q", output.Status, "closed")
+	}
+
+	if !output.HasFile {
+		t.Error("HasFile should be true")
+	}
+}
+
+func TestConvoyMeta(t *testing.T) {
+	// Test ConvoyMeta struct
+	meta := ConvoyMeta{
+		ID:        "hq-cv-abc",
+		Title:     "Code Review: PR #123",
+		Status:    "open",
+		Formula:   "code-review",
+		ReviewID:  "pr123",
+		LegIssues: []string{"gt-leg1", "gt-leg2", "gt-leg3"},
+	}
+
+	if meta.ID != "hq-cv-abc" {
+		t.Errorf("ID = %q, want %q", meta.ID, "hq-cv-abc")
+	}
+
+	if len(meta.LegIssues) != 3 {
+		t.Errorf("len(LegIssues) = %d, want 3", len(meta.LegIssues))
+	}
+}