fix(agent): py3.10 startup crash + submit-path retry; pin agent ruff to py310

The agent container (CUDA base, Python 3.10) crashed on startup with
`NameError: name 'Config' is not defined` — an earlier `ruff --fix` unquoted the
`from_env(cls) -> Config` self-reference, which is safe on CI's Python 3.14
(PEP 649 lazy annotations) but is evaluated at class-definition time on 3.10.
CI lint/compile run on 3.14, so it slipped through.

- config.py: `from __future__ import annotations` so the self-referential
  annotation is a string, never evaluated — works on 3.10 and every version.
- agent/ruff.toml: pin the agent to `target-version = "py310"` (its real runtime)
  and inherit the root rules. Ruff now flags exactly this class as F821, so CI's
  lint lane catches it instead of shipping a broken image. (CI otherwise lints on
  3.14, masking 3.10 issues.)
- client.py: submit path now retries in-place. A dedicated session with a
  urllib3 Retry (connect/read/status, 0.5s backoff, 500/502/503/504, POST) so a
  momentary blip after the GPU work is done doesn't discard it and force a full
  re-download + recompute elsewhere. A duplicate submit after a lost response is
  a harmless 409 no-op. Lease/fetch keep the plain session + loop-level backoff.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

agent/fc_agent/config.py

@@ -1,4 +1,9 @@
 """Agent config, all from env (the control container is configured at run)."""
+# Lazy annotations so the `from_env(cls) -> Config` self-reference is a string,
+# not evaluated at class-definition time — otherwise it NameErrors on the agent's
+# Python 3.10 (CI lints on 3.14, where PEP 649 hides this).
+from __future__ import annotations
+
 import os
 from dataclasses import dataclass