feat(mvp): Phase 1 control layer + API vertical slice

Implements the Phase 1 MVP from the README: a stateless control layer + HTTP
API over a centralized database, with hook-enforced test/push gates.

- DB layer: SQLAlchemy Core, one schema rendering both Postgres (BIGSERIAL /
  TIMESTAMPTZ / JSONB) and SQLite (INTEGER PK / TEXT / JSON) via portable types;
  native ON CONFLICT DO UPDATE checkmark upsert on both dialects.
- Alembic dual-dialect migrations (render_as_batch for SQLite); tests run a real
  `alembic upgrade head`.
- FastAPI: projects/agents/checkmark/log/answer/resume + shared log/context
  routes, single global bearer token, higher-trust token gating shared-context
  writes, project isolation on every route.
- Hooks (`python -m handler.hooks <event>`): Stop test gate (block on red),
  PreToolUse AskUserQuestion defer + `git push` gate (tests then throwaway
  build), Notification generic webhook (no-op without WEBHOOK_URL). Identity via
  env injected at spawn; verify is the mock seam.
- Control CLI: spawn/list/attach/kill, hard `.mise.toml [tasks.test]` gate,
  generated per-agent settings.json, identity + DATABASE_URL injected via tmux;
  tmux is the mock seam.
- 45 tests (SQLite), ruff clean. Live claude/tmux/mise spawning deferred behind
  the mocked seams.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W5ZuS5pV1NS6eKsRZHXonY

src/handler/api/routes/common.py

@@ -0,0 +1,25 @@
+"""Small route helpers shared across agent-scoped endpoints."""
+
+from __future__ import annotations
+
+from fastapi import HTTPException, status
+from sqlalchemy import Connection
+
+from ...db import repository as repo
+
+
+def resolve_agent(conn: Connection, project: str, name: str) -> dict:
+    """Fetch an agent by ``(project, name)`` or 404.
+
+    Enforces project isolation (README 3.4): the lookup is always project-scoped, so
+    there is no path that returns another project's agent by accident.
+    """
+    if repo.get_project(conn, project) is None:
+        raise HTTPException(status.HTTP_404_NOT_FOUND, detail=f"project '{project}' not found")
+    agent = repo.get_agent_by_name(conn, project, name)
+    if agent is None:
+        raise HTTPException(
+            status.HTTP_404_NOT_FOUND,
+            detail=f"agent '{name}' not found in project '{project}'",
+        )
+    return agent