feat(engine): Phase 4 — scheduler tick, gate chain, platform git, budget

The AI workforce engine's driver. One tick moves every ready task one step:
a turn, a gate, or a platform merge. The AI never runs git.

- scheduler.py: tick() over TURN_STAGES/MERGE_STAGES; selection caps
  (per-org 3, global 8); optimistic advance so two workers cannot both win;
  worktree created for code pipelines, released on completion/cancel/failure
  (kept during error_review); budget pause before any turn is spent; merge
  conflict back-off; transient-failure retries then the manager's error turn.
- gates.py: diff guard (empty/.git/CI/symlink/oversize), no-progress hash,
  tests in the sandbox, green gate that only trusts a green tests row or an
  explicit waiver. Every verdict is a gate_results row + a task.gate event.
  Fixed forbidden_paths: lstrip("./") stripped all leading dots, hiding
  .git and .github from the guard.
- container_engine.build_scheduler + worker drives the engine tick.
- ManagerOutcome.error_kind; TaskSpendRepository.spend_for_task.
- New settings engine_max_concurrent_turns, engine_max_turns_per_org,
  engine_tests_timeout_seconds, engine_max_merge_conflicts,
  engine_max_task_credits (+ .env.example).
- tests: test_engine_gates.py (13), test_engine_flow.py (11).

Checks: pytest 746 passed/1 skipped, ruff clean, mypy clean (160 files).

apps/api/tests/engine_fakes.py

@@ -225,6 +225,8 @@ class RecordingMemory:
     def __init__(self, context: str = "") -> None:
         self.context = context
         self.queries: list[dict] = []
+        self.recorded: list[dict] = []
+        self.finished: list[dict] = []
 
     async def build_context(
         self, *, organization_id: str, role: str, query: str, budget_chars: int = 2500
@@ -244,9 +246,18 @@ class RecordingMemory:
         evidence_task_id: str | None = None,
         key: str | None = None,
     ) -> str | None:
+        self.recorded.append(
+            {
+                "organization_id": organization_id,
+                "kind": kind,
+                "content": content,
+                "source": source,
+            }
+        )
         return None
 
     async def on_task_finished(self, *, organization_id: str, task) -> None:
+        self.finished.append({"organization_id": organization_id, "task": task})
         return None