feat: add session_id field to issue close/update mutations (bd-tksk)

Adds closed_by_session tracking for entity CV building per Gas Town
decision 009-session-events-architecture.md.

Changes:
- Add ClosedBySession field to Issue struct
- Add closed_by_session column to issues table (migration 034)
- Add --session flag to bd close command
- Support CLAUDE_SESSION_ID env var as fallback
- Add --session flag to bd update for status=closed
- Display closed_by_session in bd show output
- Update Storage interface to include session parameter in CloseIssue

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

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

Executed-By: beads/crew/dave
Rig: beads
Role: crew

examples/bd-example-extension-go/main.go

@@ -65,7 +65,7 @@ func main() {
 
 	// Complete
 	db.Exec(`UPDATE example_executions SET status='completed', completed_at=? WHERE id=?`, time.Now(), execID)
-	store.CloseIssue(ctx, issue.ID, "Done", "demo-agent")
+	store.CloseIssue(ctx, issue.ID, "Done", "demo-agent", "")
 
 	// Show status
 	fmt.Println("\nStatus:")