docs: fix stale Permissions-Policy scoring row and add CI Integration section

[dmchaledev]

Combines #90 and #60, rebased onto current main:

• From docs: fix stale Permissions-Policy and CSP scoring criteria in README #90: the "Headers Checked" table's Permissions-Policy row still said "presence" — stale since that check was tightened to require camera/microphone/geolocation restriction. Fixed. (The CSP row docs: fix stale Permissions-Policy and CSP scoring criteria in README #90 also touched has already been corrected by other merges since this PR was opened, so that hunk was dropped as moot.)
• From docs: add GitHub Actions CI integration examples #60: adds a "CI Integration" section with GitHub Actions examples (blocking gate, non-blocking audit, JSON artifact export). Updated the blocking-gate note to also mention the --fail-on flag added by feat(cli): add --fail-on grade option for configurable CI gate threshold #73 since this PR was originally opened.

Closes #90, closes #60.

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