Slice 1 (4/4): walking skeleton — stub runner + container entrypoint + e2e; CLAUDE.md - #4

… CLAUDE.md

Slice 1, PR 4 of 4 — the walking skeleton proving the four contracts end to end.

- panopticon.container.client.TaskServiceClient: a thin REST client used from inside
  a task container (wraps httpx; a FastAPI TestClient in tests).
- panopticon.container.entrypoint.run_task_container: the entrypoint protocol —
  register (liveness), set the slug if unset (the slug hook; slugs are decided in the
  container), run work, deregister. No Docker, no LLM in this slice.
- panopticon.sessionservice.stub_runner.StubRunner: stands in for the runner by
  running the entrypoint in-process.
- tests/test_skeleton.py: create -> register -> set slug -> transition -> history,
  end to end over REST, with liveness cleanup asserted.
- CLAUDE.md: the operating manual (determinism invariant, module map, conventions,
  glossary), per the roadmap's per-slice definition of done.

Moves httpx to runtime deps (the container client uses it).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>