feat(cli): add port ownership diagnosis before serve startup

[dggyey]

What does this PR do?

Adds a port ownership diagnostic that runs before areno serve/areno proxy startup. The diagnostic checks if the target (host, port) is available for binding. If occupied, it identifies the owning process via psutil, classifies it as an AReno child process or an unrelated external service, and prints a safe, actionable suggestion — without terminating any process.

Changes

• New areno/cli/port_diag.py: core diagnostic logic (diagnose_port(), format_diagnosis(), PortDiagnosis dataclass)
• Modified areno/cli/serve.py: integrated diagnostic before model/worker initialization
• Modified areno/cli/diagnostics.py: added areno port-check CLI command with --host/--port/--json options
• Modified areno/cli/main.py: registered port-check command in the CLI group
• New tests/test_port_diag_cpu.py: 17 CPU-only tests
• Updated docs/cli/diagnostics.rst: CLI reference documentation
• New docs/troubleshooting/port-conflict.rst: troubleshooting guide with minimal runnable example

Related issue

Fixes #228

Type of change

• ✨ New feature

How was it tested?

Automated tests

pytest tests/test_port_diag_cpu.py -v

Result: 17 passed.

Manual tests on Linux (Kaggle)

Test 1: Free port

areno port-check --host 127.0.0.1 --port 18080

Port diagnostic: 127.0.0.1:18080
  Status: available
  Suggestion: Port is available for binding.

Exit code: 0

Test 2: Occupied port — PID and process detected

areno port-check --host 127.0.0.1 --port 19000

Port diagnostic: 127.0.0.1:19000
  Status: occupied
  PID: 543
  Process: python3
  AReno child: no
  Bind error: [Errno 98] Address already in use
  Suggestion: Port 19000 is held by 'python3' (PID 543). Change the port or stop that process manually.

Exit code: 1

Test 3: areno serve auto-diagnostic before model load (port occupied)

areno serve --model-path Qwen/Qwen3-0.6B --model-hub hf --port 8000

Port diagnostic: 0.0.0.0:8000
  Status: occupied
  PID: 543
  Process: python3
  AReno child: no
  Bind error: [Errno 98] Address already in use
  Suggestion: Port 8000 is held by 'python3' (PID 543). Change the port or stop that process manually.

Exit code: 1 — no model download or loading occurred; diagnostic intercepted startup before resolve_model_ref().

Checklist

• The PR title summarizes the contribution.
• Linked the related issue in the description (if any).
• Existing tests pass (pytest tests/ -k cpu).
• New behavior is covered by tests.
• Described the test commands run and any hardware limitations.
• Public API / CLI changes are additive and backward-compatible (see CONTRIBUTING.md).

Breaking change details

No breaking changes. The port-check command is additive. In areno serve, the diagnostic runs before model initialization but only exits early when the port is occupied by an unrelated process — when the port is available, the existing serve flow is unchanged.