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/transaction_test.go

@@ -254,7 +254,7 @@ func TestTransactionCloseIssue(t *testing.T) {
 
 	// Close in transaction
 	err := store.RunInTransaction(ctx, func(tx storage.Transaction) error {
-		return tx.CloseIssue(ctx, issue.ID, "Done", "test-actor")
+		return tx.CloseIssue(ctx, issue.ID, "Done", "test-actor", "")
 	})
 
 	if err != nil {