ci: soft-skip self-review on missing key / billing

[aisona-lab]

Summary

• .github/workflows/self-review.yml: `if: ${{ secrets.ANTHROPIC_API_KEY != '' }}` so empty/missing secret skips the job (not a red check). Keep `fail-on: never`.
• `action.yml`: when `fail-on=never`, Anthropic auth/billing/empty-credits soft-skips with a warning instead of failing on operational exit 3 (root cause of prior red self-review runs).
• README / AGENTS / FEATURE_MAP: one key or no key — gate is auth engine; lazycoder LLM optional; never claim two keys. Document untrusted data + offline prove/replay.

Proof (local, no key)

```bash
env -u ANTHROPIC_API_KEY uv run pytest -q

EXIT 0 — 114 passed, 1 deselected

env -u ANTHROPIC_API_KEY uv run python scripts/corpus_cli.py prove corpus/seed.jsonl

EXIT 0 — OFFLINE_PROVE: PASS

env -u ANTHROPIC_API_KEY uv run pytest -q \
tests/test_cli.py::test_cli_replay_matches_recorded_verdict_without_model \
tests/test_decision_log.py::test_verdict_replays_from_the_record_alone

EXIT 0

uv run ruff check . && uv run black --check . && uv run mypy src

EXIT 0

```

Test plan

• pytest / prove / replay offline exit 0 without `ANTHROPIC_API_KEY`
• `test` workflow green
• `self-review` skips or soft-passes (not hard-red) when secret empty or credits empty

[github-actions]

lazycoder review — 💥 ERROR

Downloading pydantic-core (2.0MiB)
Downloading anthropic (1.3MiB)
 Downloaded pydantic-core
 Downloaded anthropic
Installed 16 packages in 35ms
error: Error code: 400 - {'type': 'error', 'error': {'type': 'invalid_request_error', 'message': 'Your credit balance is too low to access the Anthropic API. Please go to Plans & Billing to upgrade or purchase credits.'}, 'request_id': 'req_011CfXk8cStqg6xQXwuqDB3B'}

_{0 finding(s), 0 abstention(s), 0 rule error(s) — R1..R17 rubric via lazycoder}