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

internal/storage/sqlite/test_helpers.go

@@ -105,7 +105,7 @@ func (e *testEnv) AddParentChild(child, parent *types.Issue) {
 // Close closes the issue with the given reason.
 func (e *testEnv) Close(issue *types.Issue, reason string) {
 	e.t.Helper()
-	if err := e.Store.CloseIssue(e.Ctx, issue.ID, reason, "test-user"); err != nil {
+	if err := e.Store.CloseIssue(e.Ctx, issue.ID, reason, "test-user", ""); err != nil {
 		e.t.Fatalf("CloseIssue(%s) failed: %v", issue.ID, err)
 	}
 }