chore(deps): bump actions/attest from 2 to 4

[nateshpp]

What this PR does

Recreates Dependabot's #134 with the repo's snapshot regenerated, since Dependabot's own PR fails this repo's check:snapshot gate (it has no way to know this repo requires .modonome/snapshot/* regenerated alongside any tracked-file change). Same version bump, same diff to action.yml; Dependabot's #134 will be closed as superseded.

• actions/attest 2 → 4 in action.yml (both the gate-integrity receipt step and the AgentProof conformance attestation step).
• .modonome/snapshot/* regenerated to match.

Governance checklist

• npm run verify gates relevant to this diff pass locally (style, repo hygiene, snapshot freshness, attestation freshness)
• No em dashes in any tracked file
• No AI co-author lines in commit messages
• If ratchet logic changed: N/A, no ratchet files touched
• If a new AgentProof scenario: N/A
• If config schema changed: N/A
• If prompt changed: N/A

Test evidence

node scripts/snapshot.mjs . --check
Snapshot is current.

npm run check:attestation
PASS: .modonome/policy-attestation.json is current.

node scripts/check-style.mjs .
Style check passed (185 advisory warning(s), grandfathered term(s) not yet migrated).

node scripts/check-repo-hygiene.mjs
Repo hygiene check passed. No loose ends detected.