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/sqlite_bench_test.go

@@ -180,7 +180,7 @@ func BenchmarkBulkCloseIssues(b *testing.B) {
 
 	for i := 0; i < b.N; i++ {
 		for j, issue := range issues {
-			if err := store.CloseIssue(ctx, issue.ID, "Bulk closed", "bench"); err != nil {
+			if err := store.CloseIssue(ctx, issue.ID, "Bulk closed", "bench", ""); err != nil {
 				b.Fatalf("CloseIssue failed: %v", err)
 			}
 			// Re-open for next iteration (except last one)