docs: fix agent-team uninstall label and update testing section

[github-actions]

What changed

Two documentation fixes surfaced during the post-merge sweep of #69.

1. README.md — wrong label in uninstall steps

The agent-team uninstall block listed state:in-progress as the seventh label to delete. That label does not exist in the repo or in the agent-team install skill. The correct seventh label is agent-team:reviewed (the label the reviewer adds when it approves a PR), which is documented correctly in catalog/agent-team/README.md prerequisites but was wrong in the top-level README.

Before:

... `plan-needed`, `impl-needed`, `review-needed`, `done`, `blocked`, and `in-progress`.
```
**After**:
```
... `plan-needed`, `impl-needed`, `review-needed`, `done`, and `blocked`, plus `gh label delete agent-team:reviewed`.

2. CONTRIBUTING.md — outdated testing section

The testing section opened with "There is no automated test harness for skills", which is incorrect. The repo ships a three-tier test suite (tests/) that runs in CI on every PR. The section now accurately describes:

• How to run the suite (./tests/run-tests.sh)
• The three-tier structure (invariants / skill behavior / E2E)
• CI behavior and the fork-PR limitation (Tier 1 fails on forks because CLAUDE_CODE_OAUTH_TOKEN is not leaked)
• Retained the manual validation steps for lock files and grep counts

Why these weren't caught earlier

The label name discrepancy predates #69. The testing section was written before the test suite was added and was never updated. Neither affects runtime behavior — they are documentation-only errors.

Checklist

• state:in-progress → agent-team:reviewed in README.md uninstall block
• CONTRIBUTING.md testing section rewritten to match the actual test suite
• No lock files touched, no skill instructions changed

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