ci: bump actions/checkout to v6 in Claude workflows

[jnasbyupgrade]

Summary

• .github/workflows/claude-code-review.yml and .github/workflows/claude.yml still used actions/checkout@v4, which targets Node.js 20. GitHub Actions runners are deprecating Node 20, and runs of these workflows show:

  Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4.

• ci.yml was already bumped to actions/checkout@v6 (Node 24). This PR brings the two Claude workflows in line with it.

Test plan

• Confirmed the warning's source via a recent claude-code-review run log (only actions/checkout@v4 was named as the offending action; anthropics/claude-code-action@v1 was not implicated).
• After this PR triggers claude-code-review, confirm that run's log no longer shows the Node.js 20 deprecation warning.
• CI green.

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[jnasbyupgrade]

Rolling this into #39 (combining the CI-workflow changes). Closing; the actions/checkout@v4→v6 bump now lives in #39. Leaving the branch in place.