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

examples/library-usage/main.go

@@ -120,7 +120,7 @@ func main() {
 
 	// Example 8: Close the issue
 	fmt.Println("\n=== Closing Issue ===")
-	if err := store.CloseIssue(ctx, newIssue.ID, "Completed demo", "library-example"); err != nil {
+	if err := store.CloseIssue(ctx, newIssue.ID, "Completed demo", "library-example", ""); err != nil {
 		log.Fatalf("Failed to close issue: %v", err)
 	}
 	fmt.Printf("Closed issue %s\n", newIssue.ID)

examples/library-usage/main_test.go

@@ -93,7 +93,7 @@ func TestExampleCompiles(t *testing.T) {
 	}
 
 	// Close issue (from example code)
-	if err := store.CloseIssue(ctx, newIssue.ID, "Test complete", "test"); err != nil {
+	if err := store.CloseIssue(ctx, newIssue.ID, "Test complete", "test", ""); err != nil {
 		t.Fatalf("Failed to close issue: %v", err)
 	}