feat: controller-owned allocator API on control-plane DB (Closes #600)

Add gitea_allocate_next_work and allocator_service routing policy on top of
the #613 ControlPlaneDB substrate. Workers get atomic assign+lease results
(or wait/no_safe_work/terminal-path outcomes) without self-selecting work
via file locks or comment-only leases. #612 remains downstream.

Closes #600

docs/architecture/control-plane-db-substrate.md

@@ -40,9 +40,19 @@
 - **#600** must call this substrate (not file locks / comment-only leases alone) for completion.
 - **#612** must write `incident_links` here and create **Gitea issues**; the allocator assigns those issues, not raw incidents.
 
+## Allocator API (#600)
+
+Module: `allocator_service.py` · MCP tool: `gitea_allocate_next_work`
+
+- Workers call `gitea_allocate_next_work(apply=false|true)` instead of self-selecting work.
+- `apply=false` returns a dry-run selection (`outcome=preview`) with skip reasons.
+- `apply=true` reserves via `ControlPlaneDB.assign_and_lease` (atomic assignment+lease).
+- Coordination source is **always** the control-plane DB — not file locks or comment-only leases.
+- Routing follows ADR §5.3 (REQUEST_CHANGES → author, terminal path first, foreign lease → wait).
+- Raw monitoring incidents are never candidates (#612 remains downstream).
+
 ## Non-goals (intentionally deferred)
 
-- Full `gitea_allocate_next_work` routing policy (REQUEST_CHANGES → author, etc.) — **#600**
 - Sentry/GlitchTip provider adapters and auto-watchdog — **#612**
 - Gitea comment/label mirror writers (may be added by allocator tooling later)