fix(plane): ax task lifecycle -- resume, stay-up runner, delivery as completion

The first live run on the local ax cluster and ax's source (docs/runner.md,
docs/sandbox.md, internal/controller/reconciler.go, internal/server/server.go)
showed a lifecycle the task-3/4 code did not model:

- `ax apply` creates a Task Suspended (Ready=False, TaskSuspended); it runs only
  after `ax resume task`. The first resume of a new image can fail with
  DeadlineExceeded while Agent Substrate builds the golden snapshot, and ax's
  server then stores phase Failed; a later resume succeeds.
- ax has no Completed phase and never reads a command's exit back.
- The runner is PID 1 and must keep serving after the command exits; stop and
  suspend deliver SIGTERM, to be forwarded to the command's process group.
  Our runner imported the task in-process and exited, which stopped the
  container ("Container guest is stopped") and made every resume time out.
- ax's controller probes /readyz?check=workspace; the query string made our
  handler answer 404.

Runner (Req 4.1-4.4): spec.command runs as a child
`python -m openultrasast.plane.tasks.<name>` in its own process group, cwd the
first workspace, AX_METADATA_URL set; ax's metadata paths are served. After the
child exits the runner reads summary.json (a crash without one becomes failed
with the exit code and stderr tail), delivers the output directory, writes
/workspace/.ousast-state/<task>.done.json and serves until SIGTERM (forwarded
to the group, SIGKILL after OUSAST_TERM_GRACE, exit 0). A boot with the marker
never reruns the command; a failed delivery is retried alone. Workspaces are
prepared on the first boot only. AX_RUNNER_EXIT_AFTER_COMMAND=1 is the
unit-test escape.

Reconciler (Req 2.2, 2.3, 3.1-3.3): `ax resume task` after apply, retrying
DeadlineExceeded/Unavailable with backoff up to OUSAST_RESUME_TIMEOUT (900 s);
completion is the artifact delivery; Failed after a resume went through, a
vanished task, or no delivery within OUSAST_TASK_TIMEOUT (7200 s) fail the
task with ax's condition message; `ax delete task` follows. Ax gains resume()
and phase() returns the condition message. doctor moves to plane/doctor.py
(tasks.md task 0 allows the sibling) to keep reconciler.py under 500 lines.

Tests: the fake ax follows the real lifecycle (Suspended, a failing first
resume that leaves Failed, Running forever, delivery only after a resume went
through), plus Failed-with-message, vanish, resume-timeout and task-timeout
scenarios; runner tests cover the child's cwd/group/metadata URL, stay-up and
SIGTERM in a real subprocess, SIGKILL of a SIGTERM-ignoring group, and the
marker. The opt-in live test now checks resume, Running, suspend and a second
resume instead of a Completed phase. design.md and tasks.md state the
lifecycle; the module audit manifest counts plane.doctor.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01EmM1j46qgKMh3zqD1QMwEo

plane/Dockerfile.runner

@@ -4,7 +4,8 @@
 # git, curl, ax's Go runner as PID 1). This image keeps the same base and the same contract -- PID 1 is
 # /usr/local/bin/ax-task-runner, fed AX_TASK_YAML / AX_WORKSPACES_YAML, serving /healthz and /readyz on port 80 --
 # but the runner is this package's `openultrasast.plane.runner`, so a task's spec.command names a module under
-# `openultrasast.plane.tasks` and the output directory is delivered to the reconciler at exit.
+# `openultrasast.plane.tasks`, run as a child process; the output directory is delivered to the reconciler when
+# it exits, and the runner then keeps serving until SIGTERM, as ax requires of PID 1.
 #
 # Build from the repository root (the package is the context):
 #     docker build -f plane/Dockerfile.runner -t openultrasast-runner:dev .