Cache gems and Solargraph annotations across CI runs

[apiology]

This PR was written by Claude Code on behalf of @apiology.

Problem: Every CI job rebuilds Solargraph's gem annotation cache and reinstalls the bundle from scratch, because neither is carried between runs.

linting.yml is the one job that tries, and it cannot work: actions/cache/restore only reads, and nothing writes that key.

# .github/workflows/linting.yml
- name: Restore cache of gem annotations
  uses: actions/cache/restore@v4   # restore-only, so the key is never saved
  with:
    key: 2025-06-26-09-${{ runner.os }}-dot-cache-${{ hashFiles('Gemfile.lock') }}

Solution: Promote that step to actions/cache@v4, whose post-job hook saves the key; add it to the rspec, typecheck and four plugin jobs; and turn on setup-ruby's bundler-cache under the cache-version linting.yml already carried.

Warm run vs master at the merged commit:

job	master	warm
Solargraph / strong	327s	128s
rails	844s	351s
rspec	806s	450s
regression	921s	633s
rspec matrix, mean of 17	632s	366s

One run each, not a benchmark. Both caches key on Gemfile.lock, so a dependency change starts cold, and that run is slower than master since it also pays the save.