chore: stop tracking .claude/scheduled_tasks.lock (#1758)

## What
Remove `.claude/scheduled_tasks.lock` from version control and add a
`.gitignore` rule so it is never committed again.

## Why
This file is an **ephemeral Claude Code scheduler lock** — its contents
are runtime process state (`sessionId`, `pid`, `procStart`,
`acquiredAt`), not source. It was accidentally committed in #1623 and is
currently tracked on `main`.

Reported by @sychen52 in [review of
#1623](#1623 (review)).

## Changes
- `git rm --cached .claude/scheduled_tasks.lock`
- Add `.claude/scheduled_tasks.lock` to `.gitignore`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Chores**
* Updated repository configuration to exclude internal runtime lock
files from version control.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Ye Yu <yeyu@nvidia.com>

Author: yeyu-nvidia

.claude/scheduled_tasks.lock

@@ -68,3 +68,6 @@ AGENTS.override.md
 
 # Ignore SonarQube analysis
 .sonar/
+
+# Claude Code runtime lock (ephemeral process state — never commit)
+.claude/scheduled_tasks.lock