fix: drop accidental worktree submodule ref + ignore .claude/

The previous commit accidentally included `.claude/worktrees/agent-a4eda97a3744456ea`
as a 160000 gitlink because `git add -A` from the main checkout caught
the locked sub-agent worktree. Remove it from the index (the worktree
directory still exists on disk for the in-flight agent) and add
`.claude/` to .gitignore so it doesn't recur.

No code or doc impact — just hygiene.

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

Author: jackeycui7

.claude/worktrees/agent-a4eda97a3744456ea

@@ -1,5 +1,6 @@
 /target
 **/*.rs.bk
+.claude/
 
 # Legacy Python (removed in 0.3.0; kept here for reference if cherry-picking)
 __pycache__/