feat: Add LGTM auto-merge command for approved PRs

## Summary

Add a `/lgtm` (or `/merge`) comment command that enables auto-merge on a PR. Once all CI checks pass, the PR merges automatically.

## Motivation

After approval, maintainers must manually return to click merge when CI finishes. This adds unnecessary latency.

## Proposed Behavior

1. Maintainer comments `/lgtm` on an approved PR
2. Workflow enables GitHub auto-merge via `gh pr merge --auto --squash`
3. PR merges when all required checks pass; if CI fails, auto-merge is cancelled

## Implementation

A GitHub Actions workflow on `issue_comment` that:
- Verifies the commenter is an org member
- Runs `gh pr merge --auto --squash` on the PR
- Supports `/lgtm cancel` to disable auto-merge