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]>

src/openultrasast/harness.py

@@ -62,13 +62,19 @@ class SerializedHarnessConfig:
 
 
 class HarnessTraceWriter:
-    def __init__(self, path: Path) -> None:
+    def __init__(self, path: Path, *, redact: bool = True) -> None:
         self.path = path
+        self.redact = redact
         self.path.parent.mkdir(parents=True, exist_ok=True)
 
     def append(self, event: HarnessEvent) -> None:
+        line = json.dumps(asdict(event), sort_keys=True)
+        if self.redact:
+            from .redaction import redact_secrets
+
+            line = redact_secrets(line)
         with self.path.open("a") as handle:
-            handle.write(json.dumps(asdict(event), sort_keys=True) + "\n")
+            handle.write(line + "\n")
 
 
 class HarnessRuntime: