fix(ci): repair recurring sync-develop checkout failure

[neuromechanist]

Problem

sync-develop.yml ("Sync develop with main and bump") has failed on every stable release (verified on v0.8.3 and v0.8.4), leaving develop unsynced after each release. It dies at the checkout step:

##[error]A branch or tag with the name 'develop' could not be found

The full log shows the cause: under the workflow_run trigger, actions/checkout's wildcard fetch

git fetch ... origin +refs/heads/*:refs/remotes/origin/* +refs/tags/*:refs/tags/*
 * [new branch]      main        -> origin/main
 * [new tag]         v0.8.4 ...

advertises only main among heads. develop is never fetched, so ref: develop can't be resolved. (The token is valid: it authenticates and fetches main + all tags.)

Fix

Check out main (always advertised), then fetch develop by explicit named ref and check it out. A named-ref fetch requests the ref directly and is reliable, unlike the wildcard advertisement.

- name: Checkout main
  uses: actions/checkout@v4
  with:
    ref: main
    fetch-depth: 0
    token: ${{ secrets.CI_ADMIN_TOKEN }}
- name: Check out develop by explicit ref
  run: |
    git fetch origin +refs/heads/develop:refs/remotes/origin/develop
    git checkout -B develop refs/remotes/origin/develop

The remaining steps (validate main version, merge --no-ff origin/main, bump to <next-patch>.dev0, push) are unchanged and operate on the now-checked-out develop.

Verification

YAML validated; step order preserved. The real test is the next stable release: the workflow should merge main into develop and bump automatically, with no manual sync. For v0.8.4 the sync was performed manually (develop is already at 0.8.5.dev0).