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/duplicates.go

@@ -283,7 +283,7 @@ func performMerge(targetID string, sourceIDs []string) map[string]interface{} {
 	for _, sourceID := range sourceIDs {
 		// Close the duplicate issue
 		reason := fmt.Sprintf("Duplicate of %s", targetID)
-		if err := store.CloseIssue(ctx, sourceID, reason, actor); err != nil {
+		if err := store.CloseIssue(ctx, sourceID, reason, actor, ""); err != nil {
 			errors = append(errors, fmt.Sprintf("failed to close %s: %v", sourceID, err))
 			continue
 		}