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/daemon_autoimport_test.go

@@ -118,7 +118,7 @@ func TestDaemonAutoImportAfterGitPull(t *testing.T) {
 	// NOW THE CRITICAL TEST: Agent A closes the issue and pushes
 	t.Run("DaemonAutoImportsAfterGitPull", func(t *testing.T) {
 		// Agent A closes the issue
-		if err := clone1Store.CloseIssue(ctx, issueID, "Completed", "agent-a"); err != nil {
+		if err := clone1Store.CloseIssue(ctx, issueID, "Completed", "agent-a", ""); err != nil {
 			t.Fatalf("Failed to close issue: %v", err)
 		}
 
@@ -331,7 +331,7 @@ func TestDaemonAutoImportDataCorruption(t *testing.T) {
 
 	// THE CORRUPTION SCENARIO:
 	// 1. Agent A closes the issue and pushes
-	clone1Store.CloseIssue(ctx, issueID, "Done", "agent-a")
+	clone1Store.CloseIssue(ctx, issueID, "Done", "agent-a", "")
 	exportIssuesToJSONL(ctx, clone1Store, clone1JSONLPath)
 	runGitCmd(t, clone1Dir, "add", ".beads/issues.jsonl")
 	runGitCmd(t, clone1Dir, "commit", "-m", "Close issue")