docs: rewrite action READMEs to be user-facing

[baxyz]

Summary

Per feedback on the published website preview (https://fix-website.helpers4.pages.dev/action/actions/setup-pnpm/): these READMEs read as internal helpers4 dev-notes ("collapses X duplicated across every helpers4 repo's CI") rather than descriptions written for someone deciding whether to use the action. A user landing on the Marketplace or website doesn't care whether/how helpers4 itself uses it — they care what it does for them.

Rewrote the opening description of the 3 affected actions to lead with capability, not origin story:

• setup-pnpm: now opens with what it does (set up Node+pnpm in one step) and keeps the one fact that matters to a user (why not corepack — Node 26 drops it), dropping "duplicated across every helpers4 repo's CI"
• pr-status-comment: now opens with the actual value (one sticky comment instead of new ones piling up) instead of "collapses an 80-line block duplicated across every helpers4 repo"
• dispatch-with-fallback: rewritten description and both examples — swapped TRIGGANATOR_ID/PUSHINATOR_ID/action-release/helpers4/website for generic placeholder values (RELEASE_APP_ID, upstream-release, my-org/docs-site), and dropped the "only typescript having it" / "Fully generic, nothing assumes helpers4" meta-commentary — the input names (target-owner, target-repo both required) already make the genericity obvious without saying so

conventional-commits's README and the root README.md's per-action blurbs were already user-facing — no changes needed there.

One description, not two — same file serves GitHub and the website, so no ongoing duplication to maintain.

🤖 Generated with Claude Code

[github-actions]

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All checks passed!

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📋 Pipeline Status

	Job	Status
✅	🧾 Conventional Commits	passing
✅	🐚 ShellCheck	passing
✅	🧪 Action Tests	passing
✅	📄 Manifest Validation	passing
✅	📦 setup-pnpm Tests	passing
✅	💬 pr-status-comment Tests	passing

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_{🤖 Generated by @helpers4 CI • 2026-07-29}