ci: pin GitHub Actions and audit dependencies

[trionnemesis]

Summary

• pin every external GitHub Action in all four workflows to an immutable 40-character commit SHA
• retain exact release comments beside each pin so updates remain reviewable
• add a blocking dependency-audit job that audits AgentSec's project metadata with pip-audit
• add regression tests that parse every workflow and enforce the reviewed action allowlist, SHA pins, and release comments

Why

This completes the remaining supply-chain hardening work in item 3 of #1. AgentSec is a security tool, so its own CI should not execute mutable action tags and should fail when a known Python dependency vulnerability is introduced.

Reviewed pins

Action	Release	Commit
actions/checkout	v4.4.0	11d5960a326750d5838078e36cf38b85af677262
actions/setup-python	v5.6.0	a26af69be951a213d495a4c3e4e4022e16d87065
actions/upload-artifact	v4.6.2	ea165f8d65b6e75b540449e92b4886f43607fa02
actions/configure-pages	v5.0.0	983d7736d9b0ae728b81ab479565c72886d7745b
actions/upload-pages-artifact	v3.0.1	56afc609e74202658d3ffba0e8f6dda462b719fa
actions/deploy-pages	v4.0.5	d6db90164ac5ed86f2b6aed7e0febac5b3c0c03e
pypa/gh-action-pip-audit	v1.1.0	1220774d901786e6f652ae159f7b6bc8fea6d266

Security boundaries

• pip-audit receives inputs: ., so it resolves the checked-out project's runtime metadata rather than auditing unrelated packages preinstalled on the runner.
• vulnerabilities fail the job; no allow-failure or blanket ignore is configured
• no repository write permissions or secrets are added
• the optional OpenSSF Scorecard integration remains a follow-up because it requires a separate decision about additional permissions and result publication

Validation

• make check
• pytest -q --cov=agentsec --cov-report=term-missing — 80.44% coverage (72% required)
• pytest -q tests/test_workflows.py
• pip-audit . — no known vulnerabilities found
• remote branch tree matches the locally validated tree exactly

Refs #1