feat(security): security-gate (gitleaks secret-scan + pip-audit) before merge

Add a deterministic (no-LLM) security sub-gate on the deploy-staging -> deploy
edge, run FIRST (before merge-gate ORCH-043 and image-freshness ORCH-058) so it
fails cheaply before any expensive rebase/rebuild, and scans origin/main..HEAD
before rebase so a task is never blamed for a CVE introduced by an updated main.

Why: the autonomous pipeline merged branches into main with no check for a leaked
secret or a vulnerable dependency. For the self-hosting orchestrator (one shared
prod instance serving every project from a shared DB) a single leak/CVE landed in
the prod of all projects (CLAUDE.md self-hosting, section 8).

- New leaf src/security_gate.py (never-raise): gitleaks (offline, fail-closed on
  tool error => secrets guarantee is unconditional) + pip-audit (best-effort;
  unreachable CVE feed degrades fail-open + loud warning by default, strict via
  security_dep_audit_fail_closed). Verdict lives ONLY in 17-security-report.md
  YAML frontmatter (write -> read-back single source of truth); FAIL is
  authoritative; missing/broken frontmatter => fail-closed.
- check_security_gate thin wrapper registered in QG_CHECKS (lazy import, no cycle).
- _handle_security_gate wired FIRST in advance_stage deploy-staging block: FAIL ->
  rollback to development + developer-retry (cap MAX_DEVELOPER_RETRIES); task_desc
  carries verbatim findings (ORCH-046 pattern). No merge-lease release (runs before
  lease acquire). Self-hosting safe: only reads/scans/writes, never deploys.
- Conditional rollout (security_gate_enabled + security_gate_repos; empty scope ->
  self-hosting only). 6 new ORCH_SECURITY_* settings.
- Infra: pinned gitleaks Go binary in Dockerfile (+curl/ca-certificates), pip-audit
  in requirements.txt, versioned .gitleaks.toml at repo root.
- STAGE_TRANSITIONS and DB schema unchanged.

Docs: docs/architecture/README.md (marked realized), CLAUDE.md (artifact 17),
CHANGELOG.md. Tests: test_security_gate.py, test_qg_security.py,
test_stage_engine_security_gate.py + updated registry/edge snapshots.

Refs: ORCH-022

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

.env.example

@@ -146,6 +146,27 @@ ORCH_REAPER_MAX_RUNNING_S=3600
 ORCH_REAPER_FINALIZE_GRACE_S=300
 ORCH_LEASE_RECLAIM_ENABLED=true
 
+# ORCH-022: security-gate (secret-scanning + dependency audit) on the
+# deploy-staging -> deploy edge, run FIRST among the edge sub-gates. Deterministic
+# (no LLM): gitleaks (offline secret-scan, pinned Go binary in the image) + pip-audit
+# (OSV/PyPI CVE audit). Verdict in the versioned 17-security-report.md frontmatter;
+# FAIL -> rollback to development + developer-retry (cap 3). See ADR-001.
+#   GATE_ENABLED          -> global kill-switch; false -> pipeline 1:1 as before ORCH-022.
+#   GATE_REPOS            -> CSV of repos where the gate is REAL; empty -> only self-hosting.
+#   DEP_BLOCK_SEVERITY    -> CVE severity that BLOCKS (CRITICAL>HIGH>MEDIUM>LOW); below /
+#                            UNKNOWN -> warning only (anti-loop).
+#   SCAN_TIMEOUT_S        -> per external scanner call timeout.
+#   DEP_AUDIT_FAIL_CLOSED -> strict mode: unreachable CVE feed -> FAIL instead of the
+#                            default fail-open + warning (anti-loop). Default false.
+#   SECRETS_BLOCK         -> a found secret blocks (always true by default; the offline
+#                            secrets guarantee is unconditional).
+ORCH_SECURITY_GATE_ENABLED=true
+ORCH_SECURITY_GATE_REPOS=
+ORCH_SECURITY_DEP_BLOCK_SEVERITY=HIGH
+ORCH_SECURITY_SCAN_TIMEOUT_S=300
+ORCH_SECURITY_DEP_AUDIT_FAIL_CLOSED=false
+ORCH_SECURITY_SECRETS_BLOCK=true
+
 # ORCH-021: post-deploy production monitoring + degradation reaction. After the
 # terminal deploy->done transition for an applicable repo, a reserved-agent job
 # `post-deploy-monitor` (no LLM, modelled on deploy-finalizer) probes prod over a