Feat/rl check preflight

[anravich13-cloud]

feat: rl --check preflight doctor

Adds a preflight mode to run some basic checks before starting runs:

uv run rl @ config.toml --check

validates the run and exits — without launching anything and without writing
to the output directory (unlike --dry-run, which writes resolved configs).
Exit code is 0 when nothing failed (warnings allowed, so CI can gate on it)
and 1 otherwise.

What it checks

Check	What / why
client/server port match	Client base_url port must equal inference.server.port — a mismatch means the orchestrator hangs against a dead port.
inference port free	Bind-probes the server's port to catch zombie vLLMs before minutes of model loading.
parallelism	Runs the trainer's real ParallelDims validation (incl. auto-EP resolution and seq-len divisor) so it fails here instead of after launch prep.
ckpt resume	Verifies resume_step exists on disk; lists available steps if not.
disk	Free space at output_dir — FAIL < 5 GB, WARN < 100 GB (checkpoints die at first write, hours in).
wandb / hf auth	WARN if wandb is configured without credentials (headless launches hang at the login prompt); HF token is informational.
envs	Spawns each configured env server and reads its info — catches broken/missing envs.
endpoints	HTTP-probes frozen/teacher models and external inference servers .

Design notes

• Checks are pure functions (RLConfig, HostProbe) -> list[CheckResult]; the
  HostProbe seam (sockets, disk, env vars) exists so checks are unit-testable
  on CPU without real host state.
• Runs at the top of rl(), before validate_output_dir/clean_future_steps
  — preflight must never mutate the output dir (today the first GPU-count
  validation fires after the dir has been cleaned).
• Checks that don't apply (elastic client, multi-node bind probe, not
  resuming, no envs) report SKIP with a reason rather than disappearing.

Test plan

• Verified on a GPU box against examples/reverse_text/rl.toml: all-green
  report, wandb WARN correctly fired on a box without credentials, exit codes
  0/1 as expected.
• Failure modes exercised: occupied port 8000 → FAIL + hint, exit 1;
  --ckpt.resume-step 999 → FAIL listing available steps.
• --dry-run and normal launch paths untouched (flag short-circuits before
  them; check excluded from written configs).

Follow-ups (out of scope)

• Refactor rl_local to reuse these check functions at launch time so the
  launch path and --check can't drift.
• Same treatment for the sft entrypoint.
• Config migration hints for renamed/removed keys (pydantic-config hook).
• CPU unit tests for the check functions (removed from this PR to keep it
  minimal; the HostProbe seam makes them cheap to add back).

---

Note

Medium Risk
Touches the main rl() entry path and spawns real env subprocesses with network probes, but normal launches are unchanged when --check is off; optional world_size on parallel helpers is backward compatible.

Overview
Adds rl @ config.toml --check, a preflight mode that validates the run and exits before output_dir is created or cleaned. Exit 0 if no failures (warnings OK); 1 on any failure.

New RLConfig.check and prime_rl.utils.doctor run host/config checks (client/inference port alignment, local inference bind probe, trainer ParallelDims / seq-len / auto-EP via optional world_size, checkpoint resume paths, disk space, wandb/HF credentials) plus live probes (HTTP for external/frozen inference URLs; bounded parallel env-server spawns with 120s timeout, SKIP on slow startup).

rl() short-circuits to run_checks at the top; check is excluded from dumped TOML. Env start/shutdown gain configurable spawn_timeout and stronger server teardown for the doctor path.

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