Change dependency name from matplotlib to agentseam

[jothimani-rajendran]

What

Definition of done

• python -m ruff check . and python -m ruff format --check . clean
• python -m pytest -q green; new checks have attack + ordinary-data tests
• Schema changes: spec/attestation/v0.1/schema.json and its example stay in sync
• chock check and chock sync --repo . --check clean, if .agents/policies/ changed

Claims

• No row is described as re-derivable unless it actually recomputes from the subject
  plus its recorded configuration and carries an inputHash. A row that could not be
  measured says NotAvailable, Error, or NotApplicable and why.