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/epic.go

@@ -166,7 +166,7 @@ var closeEligibleEpicsCmd = &cobra.Command{
 				}
 			} else {
 				ctx := rootCtx
-				err := store.CloseIssue(ctx, epicStatus.Epic.ID, "All children completed", "system")
+				err := store.CloseIssue(ctx, epicStatus.Epic.ID, "All children completed", "system", "")
 				if err != nil {
 					fmt.Fprintf(os.Stderr, "Error closing %s: %v\n", epicStatus.Epic.ID, err)
 					continue