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

cmd/bd/update.go

@@ -40,6 +40,17 @@ create, update, show, or close operation).`,
 		if cmd.Flags().Changed("status") {
 			status, _ := cmd.Flags().GetString("status")
 			updates["status"] = status
+
+			// If status is being set to closed, include session if provided
+			if status == "closed" {
+				session, _ := cmd.Flags().GetString("session")
+				if session == "" {
+					session = os.Getenv("CLAUDE_SESSION_ID")
+				}
+				if session != "" {
+					updates["closed_by_session"] = session
+				}
+			}
 		}
 		if cmd.Flags().Changed("priority") {
 			priorityStr, _ := cmd.Flags().GetString("priority")
@@ -412,5 +423,6 @@ func init() {
 	updateCmd.Flags().StringSlice("set-labels", nil, "Set labels, replacing all existing (repeatable)")
 	updateCmd.Flags().String("parent", "", "New parent issue ID (reparents the issue, use empty string to remove parent)")
 	updateCmd.Flags().Bool("claim", false, "Atomically claim the issue (sets assignee to you, status to in_progress; fails if already claimed)")
+	updateCmd.Flags().String("session", "", "Claude Code session ID for status=closed (or set CLAUDE_SESSION_ID env var)")
 	rootCmd.AddCommand(updateCmd)
 }