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

.gitignore

@@ -12,7 +12,7 @@ d.update*
 
 # delegate-job 임시/런타임 산출물
 .mam/
-.venv/
+.venv
 __pycache__/
 *.pyc
 *.tmp