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]>

docs/threat-model.md

@@ -0,0 +1,66 @@
+# Threat model & hardening
+
+OpenUltraSAST analyzes **untrusted code**. This document states what it trusts, what
+it does not, and the controls that keep a scan safe to run in CI and safe to share.
+
+## Trust boundaries
+
+- **Scanned code is untrusted input, never executed.** Quick and standard modes read
+  source as text (regex rules, evidence ladder, optional LLM review). They never import,
+  build, or run the target. Only `deep` mode would execute target-derived code, and only
+  inside the sandbox below — and deep mode is not yet implemented (it exits with a clear
+  message rather than running anything).
+- **The ruleset and CWE policy are trusted, governed data.** They change only through
+  reviewed commits or the bounded self-improvement loop (`ousast improve`), which can
+  flip rule *status* and tune score constants but can never edit pattern text or the
+  authoritative 0–5 severity. See the self-improving cycle in the README.
+- **Provider credentials are the operator's.** The zero-dependency core makes no network
+  calls. The optional HarnessX agentic plane sends source excerpts to the configured LLM
+  provider only when a model is set in `[models]`; keys are read from the provider's
+  standard env var (`OPENAI_API_KEY` / `ANTHROPIC_API_KEY`), never written to artifacts.
+
+## Controls
+
+### Secret redaction
+Scan artifacts can quote source that contains live credentials. `redaction.py` masks
+recognizable secret shapes (provider API keys, AWS/GitHub/Slack/Google tokens, bearer
+tokens, URL-embedded credentials, PEM private keys, and `key = value` assignments for
+sensitive names) before traces (`trace/events.jsonl`) and the markdown report are
+written. On by default; disable with `[hardening] redact_secrets = false`.
+
+### Cost & CI budgets
+- **Agentic spend** is bounded per task by `[harnessx] max_cost_usd` and
+  `token_threshold` (enforced inside the HarnessX run loop) and by the per-tier hunter
+  budgets.
+- **Output size** is bounded by `[hardening] max_findings` (0 = unlimited). Truncation is
+  severity-ordered and disclosed as a `budget` degradation in the manifest.
+
+### Provider reliability
+LLM/embedding calls retry transient failures (HTTP 429/5xx, connection errors, timeouts)
+with exponential backoff; non-transient errors (e.g. 4xx, malformed JSON) fail fast.
+
+### Visible degradation & determinism
+When an optional capability is missing (the extra, a model, a provider key), the scan
+falls back to its deterministic equivalent and records a `degradations` entry in the
+manifest — it is never silently downgraded. Runs are reproducible: fixed config,
+artifact manifests, prompt hashes, and model identifiers are recorded.
+
+### MCP surface
+`ousast mcp` exposes only the ten narrow project tools. No tool runs arbitrary shell,
+Docker, or internal hunter tools, and no tool accepts a free-form command argument.
+
+## Sandbox limits (deep mode, when implemented)
+
+Dynamic analysis runs under the bounds in `[sandbox]` (`config.SandboxConfig`):
+
+| Control | Default |
+|---|---|
+| network | disabled (`network = false`) |
+| workspace | read-only (`workspace_readonly = true`) |
+| memory | 2048 MB (`memory_mb`) |
+| pids | 512 (`pids_limit`) |
+| timeout | 300 s (`timeout_seconds`) |
+
+Dynamic probes additionally declare their network scope, log commands, and capture
+artifacts; a hardened runtime (e.g. gVisor) is used when available. Until deep mode
+ships, no target code is executed at all.