deps: add graphify optional extra

[LIghtJUNction]

Summary

• add the graph optional dependency extra with graphifyy>=0.7.0
• register a requires_graphify pytest marker
• exclude Graphify-marked tests from the default pytest marker expression

Closes #6.

Validation

• python -c "import tomllib; data=tomllib.load(open(\"pyproject.toml\",\"rb\")); assert data[\"project\"][\"optional-dependencies\"][\"graph\"]==[\"graphifyy>=0.7.0\"]; assert \"requires_graphify\" in data[\"tool\"][\"pytest\"][\"ini_options\"][\"addopts\"]; assert any(m.startswith(\"requires_graphify:\") for m in data[\"tool\"][\"pytest\"][\"ini_options\"][\"markers\"]); print(\"pyproject graph extra and marker ok\")"
• python -c "import agent_notes; print(\"agent_notes import ok\")"
• git diff --check

Note: python -m pytest -q could not run in my local environment because pytest is not installed (No module named pytest).

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