Untrack .claude/, .codex, version.txt; clean up .gitignore

The release commit accidentally pulled in .claude/worktrees/ and
.codex via git add -A in build-and-push.sh. Add them to .gitignore
and remove from the index. Also drop unused version.txt and the
no-longer-used .last-branch entry.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.claude/worktrees/admiring-noether-621bfe

@@ -1 +0,0 @@
-Subproject commit 6083744b3f3e7b5ec52f5f7b935f1d7e06b97a1d

.gitignore

@@ -1,6 +1,7 @@
 .files/
 
-.last-branch
-
 __pycache__/
 *.pyc
+
+.claude/
+.codex

version.txt

@@ -1 +0,0 @@
-v0.2.8