Fix graph similarity attach and harden maintenance paths

[stevesolun]

Summary

• fix exact semantic top-k parity and new-node recompute behavior
• make ANN attach overlays authoritative for updated node incident scores
• validate overlay similarity fields at artifact and install time
• harden dashboard mutation host checks, MCP/harness install surfaces, hooks, overlay maintenance, CI graph classification, and configured Claude paths
• refresh README/docs stats

Local validation

• python -m pytest -q --no-cov <focused graph/install/dashboard/CI/config suites> (737 passed)
• python -m ruff check src hooks scripts
• python -m mypy --cache-dir %TEMP%\ctx-mypy-deep-sim-final3 src
• python -m pip check
• python src\update_repo_stats.py --check
• python src\validate_graph_artifacts.py --graph-dir graph --deep ... exact expected counts
• python scripts\ci_preflight.py --profile pr (15/15 passed)