Port sync-branches as a composite action

[roborourke]

Summary

• Adds .github/actions/sync-branches/action.yml — a composite/bash action that ports humanmade/sync-branches, replacing the deprecated node12 JS action with a gh CLI shell step that matches the conventions of every other action in this repo (no dist/, no npm dependencies)
• Adds .github/actions/sync-branches/README.md — per-action docs with a usage example and notes on pairing with resolve-composer-lock-conflict
• Adds .gitignore — ignores .worktrees/ so local worktrees don't appear as untracked files
• Updates root README.md to include both the new sync-branches and the in-flight resolve-composer-lock-conflict action entries

Behaviour preserved verbatim from the upstream JS:

• Removes the triggering label from the source PR before starting
• Fails with a comment on the source PR if the sync branch already exists
• Creates the sync branch at the source PR's head SHA
• Skips creating a duplicate PR if one already exists between the same branch pair
• Assigns the sync PR to the original PR author

What's changed vs upstream:

• Input names are snake_case to match this repo's convention (from_branch, to_branch, etc.)
• github_token defaults to ${{ github.token }} — callers only need to pass it when they need a PAT
• Uses gh api / gh pr rather than @actions/github Octokit
• No bundled dist/ — the step is a single inline bash script

Note

After this PR merges, consumers should migrate from humanmade/sync-branches@master to humanmade/hm-github-actions/.github/actions/sync-branches@<SHA> and update input names to snake_case.

Test plan

• Label an open PR "Push to Dev" — confirm a {source}-dev branch and sync PR are created, label is removed, and the sync PR is assigned to the original author
• Trigger again without deleting the sync branch — confirm a comment is posted on the source PR and the job fails
• Label a PR where the label has already been manually removed — confirm the action exits cleanly without creating a sync PR
• Pass pull_request_is_draft: "true" — confirm the sync PR is opened as a draft

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