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/transaction.go

@@ -523,13 +523,14 @@ func applyUpdatesToIssue(issue *types.Issue, updates map[string]interface{}) {
 
 // CloseIssue closes an issue within the transaction.
 // NOTE: close_reason is stored in both issues table and events table - see SQLiteStorage.CloseIssue.
-func (t *sqliteTxStorage) CloseIssue(ctx context.Context, id string, reason string, actor string) error {
+// The session parameter tracks which Claude Code session closed the issue (can be empty).
+func (t *sqliteTxStorage) CloseIssue(ctx context.Context, id string, reason string, actor string, session string) error {
 	now := time.Now()
 
 	result, err := t.conn.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)
 	}