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/refile.go

@@ -133,7 +133,7 @@ Examples:
 		// Step 7: Close the source issue (unless --keep-open)
 		if !keepOpen {
 			closeReason := fmt.Sprintf("Refiled to %s", newIssue.ID)
-			if err := result.Store.CloseIssue(ctx, resolvedSourceID, closeReason, actor); err != nil {
+			if err := result.Store.CloseIssue(ctx, resolvedSourceID, closeReason, actor, ""); err != nil {
 				WarnError("failed to close source issue: %v", err)
 			}
 			// Schedule auto-flush if source was local store