feat: add --model to community labeling commands

[balloon72]

Closes #1277

What

Adds an explicit model override for community naming in graphify cluster-only and graphify label.

Why

extract already supports --model, but community labeling only accepted --backend. For Gemini users this could fall back to the backend default model during cluster-only / label, even when extraction had used a lower-quota-pressure model such as gemini-3.1-flash-lite.

Fix

• Parse --model / --model=<name> in the shared cluster-only and label command path.
• Thread the optional model through generate_community_labels, label_communities, and _call_llm.
• Preserve existing default behavior when no model override is provided.

Test

• python -m pytest tests/test_labeling.py tests/test_claude_cli_backend.py::test_call_llm_claude_cli_branch_honours_timeout tests/test_claude_cli_backend.py::test_simple_completion_resolves_cmd_shim_on_windows -q
• python -m py_compile graphify\llm.py graphify\__main__.py tests\test_labeling.py
• git diff --check