The critic could rubber-stamp PRs that compiled and passed tests but
visibly violated project quality rules (e.g. `except Exception: pass`,
weak assertions). With nothing teaching the critic about the
manifestos, auto-merge shipped drift.

This wires the critic into the manifestos:

- `docs/manifestos/` is now the canonical home for project rules. Two
  seed manifestos (error-handling, testing) ship with the repo so the
  feature is operative on day one.
- `_critic_sdk.load_manifestos_text` reads every `*.md`/`*.txt` file
  under the canonical dir (overridable via `LOOP_MANIFESTOS_DIR`) and
  renders one block per file. `critic.review_pr` interpolates that
  block into `briefs/critic.md.tmpl` via a new `{manifestos}` slot.
- `CriticReport` gains a `manifesto_violations: list[ManifestoViolation]`
  field. `ManifestoViolation` is a typed dataclass with rule_id,
  manifesto, quote, suggested_fix, severity. Back-compat: JSON without
  the field still parses (defaults to `[]`).
- `_coerce_report` drops malformed entries (missing rule_id/manifesto),
  treats unknown severities as sev3 (defensive default), and forces
  `overall = "request_changes"` whenever any sev1 violation appears —
  even if the model emitted `approve`.
- `critic_actions.plan_actions` blocks auto-merge on any sev1 manifesto
  violation, labels the PR `critic:blocking` + `critic:manifesto-violation`,
  and posts a manifesto-violation summary comment via the gh client.
  sev2/sev3 violations are reported but not blocking.
- Prompt template now teaches the model to scan for manifesto rule
  violations and emit the new JSON shape.

Tests (`tests/test_critic_manifesto_check.py`):
- happy-path: manifesto text rendered into prompt; violations field
  parses; back-compat with absent field; sev1 flips overall;
  sev2 leaves it alone.
- adversarial: malformed entry skipped; unknown severity → sev3;
  wrong-type field ignored.
- gate: sev1 violation blocks; sev2-only does not.

Existing `tests/test_briefs.py::test_render_critic_brief_*` adjusted to
pass the new required `manifestos` kwarg.