ci: warn when changesets don't cover modified packages [SIMULATION]

[turnipdabeets]

💡 Motivation and Context

Draft / simulation — do not merge as-is.

Adds a non-blocking CI workflow that compares packages modified in a PR against packages declared in any changeset added/modified in that PR, and posts a sticky comment if they don't line up. Designed to catch the class of mistake that caused #491 → #507: changeset frontmatter pointing at the wrong package.

This PR also intentionally simulates that mistake so the workflow's comment can be observed end-to-end:

• Code change (the marker file) is in posthog-android-gradle-plugin/
• Changeset declares "posthog-android": patch
• The workflow should post a comment flagging both directions of the mismatch

When validation is done, the simulation files (.changeset/coverage-check-simulation.md and posthog-android-gradle-plugin/SIMULATION_DELETE_BEFORE_MERGE.md) get deleted, leaving just the workflow + script.

What the workflow does

• Triggers on pull_request against main.
• Parses pnpm-workspace.yaml and each package's package.json to map directories → package names.
• Diffs against the merge base; collects packages where any non-CHANGELOG.md/package.json source file changed.
• Reads frontmatter of any .changeset/*.md added or modified in the PR.
• Renders a markdown report and posts (or updates) a sticky PR comment via marocchino/sticky-pull-request-comment.
• Never fails the build — exits 0 in every case. Skipping the check = don't add a changeset.

💚 How did you test it?

• Dry-ran scripts/check-changeset-coverage.mjs locally on this branch — it correctly identified posthog-android-gradle-plugin as modified-but-undeclared and posthog-android as declared-but-unmodified.
• The simulation in this PR is the integration test — once CI runs, the sticky comment should appear with the same content.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file
• Added the "release" label to the PR to indicate we're publishing new versions for the affected packages

(The changeset present in this PR is part of the simulation, not a real release. No release label.)

Follow-ups before this is mergeable

• Pin marocchino/sticky-pull-request-comment@v2 to a SHA (the rest of the codebase pins third-party actions to SHA).
• Delete the two simulation files.
• Optional: add the workflow to required-checks-but-warn-only docs if PostHog has any.

[github-actions]

⚠️ Possible changeset mismatch — modified and declared packages differ

This is informational — the PR is not blocked. Click the triangle above to collapse, or push a fix and this comment will auto-delete.

Modified in this PR but not in any changeset:

• posthog-android-gradle-plugin

If this package should ship the change, add it to the changeset frontmatter:

---
"posthog-android-gradle-plugin": patch
---

Declared in a changeset but no source files modified:

• posthog-android

Double-check this is intentional — for example, releasing a previously-merged change.

Changesets in this PR:

• posthog-android — patch