Make the rubocop CI job blocking again, automate rubocop-todo

[apiology]

Summary

• Bump dev-dependency RuboCop from 1.80.0 to 1.89.0. 1.80.0 had a bug where per-file AllCops:Exclude entries didn't remove files from the scan target list, so spec/fixtures/invalid_{byte,utf8,node_comment}.rb (fixtures with intentionally invalid encoding, used by spec/source_spec.rb and spec/source_map/mapper_spec.rb to test Solargraph's own error handling) always failed Lint/Syntax regardless of Exclude config — confirmed this doesn't reproduce on 1.89.0, and no config-based fix exists (RuboCop refuses any config for Lint/Syntax at all).
• Consolidate the three fixture exclude entries in .rubocop.yml into one glob.
• Fix the todo-sync step in .github/workflows/linting.yml: rubocop --auto-gen-config's exit code reflects the total offense count, not just newly-introduced offenses, so under GitHub Actions' default bash -e it aborted before ever reaching the git-diff check that's meant to be the actual pass/fail signal.
• Remove continue-on-error: true from both rubocop_todo job steps (marked @todo Temporary, expect to revert in 0.60 — already at 0.60.2) and regenerate .rubocop_todo.yml against 1.89.0.
• On push to master only: if RuboCop is clean, auto-regenerate .rubocop_todo.yml and push the result if it shrank (i.e. someone fixed a todo'd offense without removing its entry), so that cleanup no longer has to be done by hand. This can't extend to PRs — GITHUB_TOKEN can't push to a fork's branch, and most PRs on this repo come from forks — so PRs keep the existing fail-with-diff behavior asking the contributor to regenerate and push it themselves.

Test plan

• bundle exec rubocop -c .rubocop.yml — 386 files inspected, no offenses detected
• Simulated the todo-sync step's git-diff logic locally — clean, no diff
• bundle exec rspec — 1623 examples, 0 failures (60 pre-existing pending)
• CI green on this PR (.rubocop_todo.yml, rubocop, overcommit all pass)
• Auto-update step verified on an actual push to master (can't be exercised from a PR)

🤖 Generated with Claude Code

https://claude.ai/code/session_018s7rWmpEoZpj4o3To7F286