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.

claude_bottle/git_gate.py

@@ -163,9 +163,15 @@ def git_gate_render_entrypoint(upstreams: tuple[GitGateUpstream, ...]) -> str:
         "  keyfile=/git-gate/creds/${name}-key",
         "  hostsfile=/git-gate/creds/${name}-known_hosts",
         "",
-        "  chmod 600 \"$keyfile\"",
+        # `|| true`: PRD 0018 chunk 3+ bind-mounts these RO from the
+        # host, so chmod-syscalls fail with EROFS. The files already
+        # have the right perms on the host (SSH requires 0600 to load
+        # the key in the first place), so the chmod is best-effort
+        # cleanup for the legacy docker-cp path where the file
+        # landed at the host's umask perms.
+        "  chmod 600 \"$keyfile\" 2>/dev/null || true",
         "  if [ -f \"$hostsfile\" ]; then",
-        "    chmod 600 \"$hostsfile\"",
+        "    chmod 600 \"$hostsfile\" 2>/dev/null || true",
         "  fi",
         "",
         "  repo=/git/${name}.git",