feat(hardening): Phase 15 — redaction, retry/backoff, budgets, threat-model docs

Release-readiness hardening.

- redaction.py (zero-dep): redact_secrets masks provider API keys, AWS/GitHub/Slack/
  Google tokens, bearer tokens, URL-embedded credentials, PEM private keys, and
  sensitive key=value assignments. Wired into the trace writer and the markdown report
  (on by default), so scan artifacts never leak a credential quoted from source.
- provider/openrouter.py: call_with_retry — exponential backoff on transient failures
  (HTTP 429/5xx, connection errors, timeouts); non-transient errors fail fast. Both the
  chat and embedding clients use it (max_attempts/retry_base_delay configurable).
- config [hardening]: redact_secrets (default true) + max_findings (bounded-CI output
  cap, 0 = unlimited; truncation is severity-ordered and disclosed as a `budget`
  degradation). Agentic spend stays bounded by [harnessx] max_cost_usd/token_threshold.
- docs/threat-model.md (trust boundaries, controls, sandbox limits) + docs/examples.md
  (six end-to-end walkthroughs); README "Security & hardening" section links both.
- CI lint/typecheck/test/detection-gate/dagger gates already existed.

tests/test_redaction.py, test_provider_retry.py, test_hardening_pipeline.py + config
tests. 206 passed, 4 skipped; ruff + mypy clean; detection gate PASS.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

README.md

@@ -383,6 +383,17 @@ constraints, retrieval filters, skill routing, and benchmark-miss triage. Those
 `RankingCalibration` fields are already produced and will be consumed directly
 once the language-aware LLM hunters land (`standard_security_harness`).
 
+## Security & hardening
+
+OpenUltraSAST treats scanned code as **untrusted input and never executes it** (quick
+and standard modes). Scan artifacts are scrubbed of credentials before they are written
+(`[hardening] redact_secrets`, on by default), agentic spend and output size are bounded
+(`[harnessx] max_cost_usd`/`token_threshold`, `[hardening] max_findings`), provider calls
+retry transient failures with backoff, and missing capabilities degrade visibly in the
+manifest rather than silently. See [docs/threat-model.md](docs/threat-model.md) for the
+full trust boundaries and sandbox limits, and [docs/examples.md](docs/examples.md) for
+end-to-end walkthroughs.
+
 ## Development
 
 ```bash
@@ -390,5 +401,6 @@ uv run pytest                 # tests, incl. the 90/10 detection gate
 uv run ruff check .           # lint
 uv run ruff format --check .  # format
 uv run mypy src/openultrasast # types
+uv run python -m openultrasast.gate  # standalone detection gate
 uv run python dagger/ci.py    # full containerized CI pipeline
 ```