Closes : #2484

Summary

This PR implements automated plugin labelling and milestone assignment for pull requests in the WordPress Performance monorepo, as requested in the linked issue.

Changes

.github/workflows/auto-label-milestone.yml (new file)

A pull_request_target workflow that runs on every PR opened, synchronize, and reopened event against trunk, release/**, and feature/** branches.

Label synchronisation

• Fetches all files changed in the PR via the GitHub API (paginated).
• Maps each changed path under plugins/<slug>/ to its corresponding [Plugin] … label using a hardcoded slug → label table derived from each plugin's Plugin Name header.
• Adds any plugin labels not yet present on the PR.
• Removes plugin labels that are currently on the PR but no longer correspond to a changed path (e.g. after a force-push that drops plugin-specific changes).

Milestone assignment

• Counts changed lines (additions + deletions) per plugin to identify the dominant plugin.
• Finds the open milestone whose title starts with <dominant-plugin-slug> (convention: embed-optimizer n.e.x.t).
• Assigns that milestone to the PR, replacing the previous one if the dominant plugin has changed.

The full slug → label mapping:

bin/plugin/commands/changelog.js (updated)

Implements the bonus changelog enhancement described in the issue.

• Adds PLUGIN_SLUG_TO_LABEL_MAP constant (same mapping as the workflow).
• Adds an optional --plugin-label / -l CLI argument. When omitted the plugin label is automatically derived from the milestone title by extracting the leading slug (e.g. embed-optimizer n.e.x.t → embed-optimizer → [Plugin] Embed Optimizer).
• Adds fetchCrossPluginPullRequests() which queries the GitHub API for closed, merged PRs that carry the plugin label and are assigned to any currently-open milestone.
• Updates fetchAllPullRequests() to merge cross-plugin PRs into the primary result set, deduplicating by PR number. This ensures a PR that touches embed-optimizer and image-prioritizer but was milestoned under embed-optimizer n.e.x.t will also appear in the image-prioritizer changelog automatically.

Use of AI Tools

I used copilot for scoping and PoA and documentation.