[apiology]

Title: [regression fix] Run CI on ubuntu-24.04 instead of ubuntu-latest

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

Problem:

Since ubuntu-latest became Ubuntu 26.04, CI fails before running any repo code:

Image: ubuntu-26.04
# rspec (3.2 .. 4.0), rubocop, typecheck, plugins: Set up Ruby / Install gems
checking for -lgit2... *** extconf.rb failed ***
mkmf.rb:490:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
An error occurred while installing rugged (1.9.6), and Bundler cannot continue.
  undercover was resolved to 0.8.5, which depends on rugged

# rspec (3.1, 3.10.4)
Error: Unavailable version 3.1.7 for ruby on ubuntu-26.04

Every Ruby check fails on every PR, whatever it changes.

Solution:

Name ubuntu-24.04 as the runner for every job, where all of these jobs passed on 2026-10-19.

Not done instead:

• A newer ruby/setup-ruby: the failing jobs already ran v1 = v1.331.0, the latest release.
• Installing build packages: libgit2's cmake step found every library it looked for; rugged's own CI fails the same way on 26.04 (libgit2/rugged#1016, test on ubuntu 24.04 and 26.04).
• Ruby 3.1 has no 26.04 build because it does not compile with GCC 15 (ruby/setup-ruby#922, add Ubuntu 26.04 support).

Moving to 26.04 can follow once rugged builds there and 3.1 leaves the matrix.