chore: stop tracking .claude/scheduled_tasks.lock

This machine-local Claude Code runtime lock was committed long ago in
e3823ce4 (shared main history) and later removed on upstream/main. Our
fork's main never got that removal, so the branch still carried it and
the PR diff was re-introducing it into upstream/main.

Untrack it (`git rm --cached`, local copy kept) and gitignore it so it
never returns. Since the base (upstream/main) already lacks the file,
removing it at the branch tip drops it from the PR diff entirely.

.claude/scheduled_tasks.lock

@@ -1 +0,0 @@
-{"sessionId":"82cd2c6b-7874-4cfa-9d00-297f91d81b98","pid":2450899,"procStart":"26101142","acquiredAt":1781931933331}

.gitignore

@@ -126,3 +126,6 @@ tests/load/storage/
 # `cargo bench --features bench`; drop real photos here (same filenames) to
 # benchmark against real data. (target/ already ignored, covers the JSON dump.)
 benches/corpus/
+
+# Claude Code scheduled-tasks runtime lock — machine-local, never committed.
+.claude/scheduled_tasks.lock