fix(repo): untrack the committed .venv symlink and ignore .venv as a file or directory

d34b532 committed `.venv` as a symlink to
/Users/godopu16/PuKi/laa/canary_projects/multi-agent-mux/.venv, the merge
worktree's preparation link. `.gitignore` had `.venv/`, which matches
directories only, so the link was not ignored. Updating a checkout to main
then silently replaces its existing (ignored) .venv directory with that
link: a broken path on other hosts and a self-referencing link in the
original checkout. Untrack it and ignore `.venv` in both forms, which also
covers the worktree preparation link of the #21 operating model.

Also correct the release date at the top of VERSIONS.md to 2026-09-14 to
match the v4.6.0 entry. No code change; the version stays 4.6.0.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PfoijjCS2U6TQPFuxAE5Pn

VERSIONS.md

@@ -7,7 +7,7 @@
 ## 📌 현재 버전 개요 (Current Release)
 
 - **프레임워크 버전**: `v4.6.0`
-- **최신 릴리스 일시**: 2026-09-13 (KST)
+- **최신 릴리스 일시**: 2026-09-14 (KST)
 - **기준 브랜치**: `main`
 - **핵심 아키텍처**:
   - **Terminal 상태 단방향 불변성 (`engine.py`, Issue #6 Phase 1)**: `completed`, `error`, `cancelled` 터미널 상태 진입 후 후속 이벤트나 업데이트에 의한 덮어쓰기를 엄격히 차단(Terminal Immutability). `registry.py close` 서브커맨드 및 fail-closed 조기 종료 도입.