[codex] fix: restrict graph commands to graphify-out

[aa-matsunari]

Summary

• apply validate_graph_path to CLI graph commands and MCP graph loading
• add regressions for blocked paths outside graphify-out

Changed files

• graphify/main.py
• graphify/serve.py
• tests/test_serve.py
• tests/test_pipeline.py

Impact

• prevents graph query tooling from reading arbitrary JSON files outside the project graph directory

Tests

• /private/tmp/graphify-review/.venv/bin/python -m pytest /private/tmp/graphify-review-pr6/tests/test_serve.py /private/tmp/graphify-review-pr6/tests/test_pipeline.py