feat(launch): switch start to docker compose project per bottle

PRD 0018 chunk 3. Each instance is now one `docker compose` project:

  - launch.py renders the compose spec via chunk-1's
    bottle_plan_to_compose, writes it to state/<slug>/docker-compose.yml,
    `docker compose up -d`s, and (on teardown) dumps
    `docker compose logs --no-color --timestamps` to
    state/<slug>/compose.log before `docker compose down`.
  - Networks are pre-created (`docker network create --internal` +
    user-defined bridge) so pipelock yaml can know the internal CIDR
    before compose-up. Compose references them with `external: true`;
    the launch step's ExitStack still owns network removal.
  - Agent still runs `sleep infinity`; claude reaches it via
    `docker exec -it` exactly like before (per the PRD's resolved
    TTY question).
  - metadata.json grows a `compose_project` field so dashboard /
    cleanup tooling can derive compose invocations without
    re-deriving the slug.

Security follow-ups from chunk-2 review:

  (b) CA private keys: pipelock + egress ca-key.pem land at 0o600
      explicitly. The mitmproxy cert+key concat stays 0o644 because
      the egress container's uid-1000 user reads it through the
      bind mount; parent dir at 0o700 still restricts host-side
      reach.
  (c) Apply atomicity: egress_apply + pipelock_apply switch from
      `docker cp` to host-side write-temp-then-rename on the
      bind-mount source. POSIX rename is atomic on the same
      filesystem, so a sidecar SIGHUP racing the apply can't see
      a half-written routes.yaml / pipelock.yaml.

Per-sidecar Docker{Sidecar}.start/stop methods stay in place — the
integration test suite drives them directly to validate each image
in isolation, which is still useful. launch.py no longer calls
them; a follow-up chunk can prune if the integration tests move to
the compose lifecycle.

git-gate entrypoint's chmod 600 on the keyfile + known_hosts now
tolerates EROFS (`|| true`) — the host SSH key is already 0600
(SSH refuses to load otherwise), so the inside-container chmod
was already a no-op in the docker-cp path and now just needs to
not error on the read-only bind mount.

422 unit tests pass; supervise integration test passes; end-to-end
`./cli.py start implementer` brings up the project, attaches,
captures full merged logs on teardown, and reaps all containers +
networks.

tests/unit/test_compose.py

@@ -176,19 +176,20 @@ class TestProjectAndNetworks(unittest.TestCase):
         spec = bottle_plan_to_compose(_plan())
         self.assertEqual(f"claude-bottle-{SLUG}", spec["name"])
 
-    def test_internal_network_is_internal(self):
+    def test_internal_network_marked_external(self):
+        # Chunk 3 pre-creates networks with `docker network create
+        # --internal` so pipelock can know the CIDR before compose-up.
+        # Compose references the network by name with `external: true`.
         spec = bottle_plan_to_compose(_plan())
         net = spec["networks"]["internal"]
         self.assertEqual(f"claude-bottle-net-{SLUG}", net["name"])
-        self.assertTrue(net["internal"])
+        self.assertTrue(net["external"])
 
-    def test_egress_network_is_external_bridge(self):
+    def test_egress_network_marked_external(self):
         spec = bottle_plan_to_compose(_plan())
         net = spec["networks"]["egress"]
         self.assertEqual(f"claude-bottle-egress-{SLUG}", net["name"])
-        # No `internal:` key on the egress network — defaults to a
-        # normal user-defined bridge.
-        self.assertNotIn("internal", net)
+        self.assertTrue(net["external"])
 
 
 class TestPipelockAlwaysPresent(unittest.TestCase):