ci: update checkout to v7

[steipete]

Summary

• update actions/checkout from v6 to stable v7
• retain the existing GitHub-hosted ubuntu-latest workflow and Node 22 test configuration

Upstream: https://github.com/actions/checkout/releases/tag/v7.0.0

Proof

• workflow YAML parse and actionlint
• scripts/validate-skills
• skills/clawsweeper-status/scripts/clawsweeper-status.test.sh
• git diff --check
• autoreview: clean, no accepted/actionable findings
• Public Model Identifier Gate: PASS; no model identifiers added

The PR's own CI run is the live workflow proof for the v7 action.

[steipete]

Landed as 2060ff0f.

Proof:

• Local: workflow YAML parse, actionlint, scripts/validate-skills, ClawSweeper helper tests, and git diff --check passed.
• Review: Codex autoreview reported no accepted/actionable findings.
• Live workflow: smoke passed using actions/checkout@v7 on exact head ca2218ef8857e346c5a4bbf4b75c2c64b4505e2e; GitGuardian also passed.
• Public Model Identifier Gate: PASS; no model identifiers added.

Caveats: none.