ci: harden GitHub Actions workflows

[BleedingDev]

Summary

• add explicit least-privilege workflow permissions
• set checkout persist-credentials: false where jobs only need read access
• move skip-CI output interpolation through environment variables before shell use
• replace ad hoc global Corepack setup with the pinned pnpm/action-setup action already used by this repository

Verification

• uvx zizmor -q --format json --no-progress --no-exit-codes --collect=workflows .github/workflows
  • before: 29 findings, H0/M18/L6/I5
  • after: 0 findings
• /Users/satan/side/experiments/skills/actionlint/scripts/run_actionlint.sh /Users/satan/worktrees/modernjs-zizmor-actions
• git diff --check

[changeset-bot]

⚠️ No Changeset found

Latest commit: 6f7e2ca

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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