feat(webui): read-only workflow policy & guardrail visibility (Closes #646)

Phase 3 child of the Web Console epic #631. Operators can now see the active
workflow policy/guardrail configuration from the console instead of reading the
repo tree.

- webui/policy_inventory.py (new): redacted, machine-readable guardrail
  inventory. One row per major guardrail (role separation/RBAC, lease rules,
  worktree binding, merge confirmation, redaction, contamination, allocator
  policy, audit logging, mutation gating) with source pointers (file/module/doc)
  and a compact active projection from the existing safe policy accessors.
  Fail-soft per entry; whole payload run through console_redaction before emit;
  diff vs documented default where feasible.
- webui/policy_views.py (new): HTML cards with source pointers, active config,
  and the documented-default diff; read-only page copy, no forms.
- webui/app.py: register GET /policy and GET /api/v1/policy (additive).
- webui/layout.py: add Policy nav item.
- tests/test_webui_policy_visibility.py (new): guardrail presence + source
  pointers (AC1), redaction incl. planted-secret masking and scan_for_secrets
  (AC2/AC3), read-only page + no-mutation routes (AC4), fail-soft rendering.
- docs/webui-local-dev.md: route table + read-only policy-visibility section.

Read-only throughout; no policy editing, no gate-weakening toggle, secrets
redacted.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

webui/layout.py

@@ -8,6 +8,7 @@ NAV_ITEMS = (
     ("/projects", "Projects"),
     ("/prompts", "Prompts"),
     ("/runtime", "Runtime"),
+    ("/policy", "Policy"),
     ("/audit", "Audit"),
     ("/worktrees", "Worktrees"),
     ("/leases", "Leases"),