fix(worker): clear error on a taken dashboard port; ordered setup output

[halfaipg]

Follow-up to #40, from verifying v0.3.10 on a 2x RTX 6000 vLLM host where a second worker ran next to the live one.

Problems

• Taken dashboard port: in console mode the worker runs inside the dashboard app. When port 7861 was already in use, uvicorn failed to bind, the worker task was cancelled and the process exited, after --setup had already printed ok.. The readiness probe also connected to the other worker's dashboard, so the log said Dashboard: http://localhost:7861 for a server this process never started.
• Jumbled --setup output: stdout is block-buffered when it is not a TTY (pipes, nohup, journald), so ok. and the setup summary printed after the worker's log lines. That made it look as if the worker started before setup finished.

Changes

• cli.main checks the dashboard port before starting in console mode. If it's taken, it exits 1 with Error: dashboard port 7861 is already in use … grid-inference-worker --port 7862.
• The desktop window (GUI) keeps the existing behaviour and opens the already-running dashboard, so a second double-click doesn't vanish silently.
• The probe uses SO_REUSEADDR like uvicorn (not on Windows), so a TIME_WAIT socket from a just-restarted service doesn't count as "in use".
• stdout and stderr are line-buffered at startup.
• DOX: inference_worker/AGENTS.md and tests/AGENTS.md updated.

Verification

• pytest tests/ passes: 171 passed, 1 skipped, including 4 new tests (real bound socket, console exit with hint and nothing started, GUI still opens the existing dashboard, line buffering).
• node --test tests/onboarding-ui.test.mjs passes: 32/32.
• Live run with the real worker holding :7861 and stdout piped:

    Checking http://127.0.0.1:8000… vLLM.
    Validating flash-next… ok.
    ...
    Saved to /home/aipg/grid-worker-src/.env
    Error: dashboard port 7861 is already in use, most likely by another worker on this machine.
    Start this worker on a free port: grid-inference-worker --port 7862
  

  Exit code 1.

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