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

internal/storage/sqlite/queries.go

@@ -664,6 +664,8 @@ var allowedUpdateFields = map[string]bool{
 	"estimated_minutes":   true,
 	"external_ref":        true,
 	"closed_at":           true,
+	"close_reason":        true,
+	"closed_by_session":   true,
 	// Messaging fields
 	"sender": true,
 	"wisp":   true, // Database column is 'ephemeral', mapped in UpdateIssue
@@ -1070,8 +1072,9 @@ func (s *SQLiteStorage) ResetCounter(ctx context.Context, prefix string) error {
 	return nil
 }
 
-// CloseIssue closes an issue with a reason
-func (s *SQLiteStorage) CloseIssue(ctx context.Context, id string, reason string, actor string) error {
+// CloseIssue closes an issue with a reason.
+// The session parameter tracks which Claude Code session closed the issue (can be empty).
+func (s *SQLiteStorage) CloseIssue(ctx context.Context, id string, reason string, actor string, session string) error {
 	now := time.Now()
 
 	// Update with special event handling
@@ -1086,9 +1089,9 @@ func (s *SQLiteStorage) CloseIssue(ctx context.Context, id string, reason string
 	// 2. events.comment - for audit history (when was it closed, by whom)
 	// Keep both in sync. If refactoring, consider deriving one from the other.
 	result, err := tx.ExecContext(ctx, `
-		UPDATE issues SET status = ?, closed_at = ?, updated_at = ?, close_reason = ?
+		UPDATE issues SET status = ?, closed_at = ?, updated_at = ?, close_reason = ?, closed_by_session = ?
 		WHERE id = ?
-	`, types.StatusClosed, now, now, reason, id)
+	`, types.StatusClosed, now, now, reason, session, id)
 	if err != nil {
 		return fmt.Errorf("failed to close issue: %w", err)
 	}