agents: Add a `--comments-only` flag to `pr-status` script, trim down context bloat in AGENTS.md (#98781)
- The `pr-status` can be very slow if you have a lot of test failures on
a PR. If the user asks a targeted question about PR review comments,
just fetch that information, it's much faster.
- Context bloat: Rely on the agent to pull in the `pr-status-triage`
skill if they need it, don't bloat the AGENTS.md, except to mention the
skill.