agent-team: implementer writes a pipeline-state table at the top of the PR; reviewer updates it

[verkyyi]

Summary

Pipeline state is visible on the issue (via <!-- agent-team:* --> comment markers) but completely invisible on the PR — the place where a human spends most of their review time. The reviewer posts a summary only on the issue, only on approve. Humans watching the PR cross-reference the issue thread to figure out where things stand.

Proposal

Implementer writes a state table as the first section of the PR body, and the reviewer updates it (via update-pull-request-body safe output) on each decision:

<!-- agent-team:pr-state -->
| Stage | Status | Run |
|---|---|---|
| Spec | ✅ | [link] |
| Plan | ✅ | [link] |
| Impl | ✅ (iter 3) | [link] |
| Review | ⏳ | — |
<!-- /agent-team:pr-state -->

Acceptance

• Implementer populates the table on PR create.
• Reviewer updates the Review row on approve/kickback and the Impl row's iteration counter on kickback.
• Uses markers so re-renders are idempotent and don't duplicate the section.