pr_merge was the only tool still advertising `index` as its id field while
every other issue/PR tool uses the canonical `number` (#38). Flipped its
schema property + required + struct field/tag `index` -> `number`; the existing
`index`->`number` shim keeps legacy `index` callers working, so no shim change
was needed (no canonical-`index` tool remains). Now the id arg is `number`
uniformly across all per-issue/PR tools.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>