Wire the quality + testing manifestos into the worker SDK system prompt
so every dispatched worker reads the project's house rules BEFORE it
sees the issue body. Previously, workers wrote PRs that violated rules
(missing tests, no incremental commits, ignored exit checklist) because
QUALITY.md/TESTING.md were not in-context at generation time.

What
----
- New `forge_loop.manifestos` module:
  * `load_manifestos(repo)` — discovery from `<repo>/.forge/*.md`.
    Whitespace-only files treated as missing. Unreadable / non-utf8
    files swallowed (corrupt manifesto MUST NOT crash worker).
  * `render_manifesto_block(bundle)` — formats the MANIFESTO prefix
    with `QUALITY RULES YOU MUST FOLLOW:` and `TESTING RULES YOU MUST
    FOLLOW:` subsections, quality first. Both missing → empty string
    (back-compat: byte-identical pre-feature prompt).
  * `inject_into_brief(brief, bundle)` — prepends the block ahead of
    any task-specific content.
  * Git-blob-style sha1 of each rendered file recorded for telemetry
    audit (parity-tested against `git hash-object`).

- `worker.make_brief` gains `manifesto_bundle=` kwarg and prepends
  the block when present. `WorkerOutcome` gains a `manifesto_sha`
  field (`{"quality": <sha>, "testing": <sha>}` or `None` per side).

- `run_worker` discovers the bundle once per dispatch from the repo
  checkout, threads it into both the brief renderer AND the outcome
  telemetry payload. Also wired through the iteration-loop
  `brief_override` path so round 2 sees the same rules.

Why
---
House rules drift the moment they aren't in-context. Injecting them
at the top of the system prompt frames every subsequent decision the
worker makes — including the exit-checklist + commit-discipline rules
that previously caused mid-session quits with uncommitted work.

Tests (tests/test_worker_manifesto_injection.py)
- Discovery: empty repo, both present, whitespace-only, binary garbage.
- Rendering: both, only-quality, only-testing, whitespace-as-missing.
- Injection: block precedes task-marker; empty bundle is a no-op.
- Telemetry: sha pairs round-trip on outcome for both/partial/none.
- Adversarial: corrupt manifesto → worker still dispatches, null sha.
- Sha parity: implementation matches `git hash-object` byte-for-byte.

Out of scope (per spec)
- Critic prompt injection (separate follow-up).
- Per-ticket manifesto overrides / templating in manifesto bodies.
- Dashboard surfacing of manifesto sha.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>