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/list_test.go

@@ -263,7 +263,7 @@ func TestListQueryCapabilitiesSuite(t *testing.T) {
 	}
 
 	// Close issue3 to set closed_at timestamp
-	if err := s.CloseIssue(ctx, issue3.ID, "test-user", "Testing"); err != nil {
+	if err := s.CloseIssue(ctx, issue3.ID, "test-user", "Testing", ""); err != nil {
 		t.Fatalf("Failed to close issue3: %v", err)
 	}