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/git_sync_test.go

@@ -71,7 +71,7 @@ func TestGitPullSyncIntegration(t *testing.T) {
 	issueID := issue.ID
 
 	// Close the issue
-	if err := clone1Store.CloseIssue(ctx, issueID, "Test completed", "test-user"); err != nil {
+	if err := clone1Store.CloseIssue(ctx, issueID, "Test completed", "test-user", ""); err != nil {
 		t.Fatalf("Failed to close issue: %v", err)
 	}