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_sync_branch_test.go

@@ -734,7 +734,7 @@ func TestSyncBranchIntegration_EndToEnd(t *testing.T) {
 	}
 
 	// Agent B closes the issue
-	store2.CloseIssue(ctx, issueID, "Done by Agent B", "agent-b")
+	store2.CloseIssue(ctx, issueID, "Done by Agent B", "agent-b", "")
 	exportToJSONLWithStore(ctx, store2, clone2JSONLPath)
 
 	// Agent B commits to sync branch