feat(rspec): add RSpec test runner with compact output (Rails/Ruby)

[deril]

Summary

• New command: rtk rspec — compact RSpec output for Rails/Ruby test suites
• Token savings: 60-90% by parsing --format json output and showing only what matters
• Hook rewrites: rspec, bin/rspec, bundle exec rspec, bundle exec rails spec → rtk rspec
• mise.toml: pin Rust toolchain for contributors using mise

How it works

RSpec's --format json is injected automatically (unless the user already passes --format). The JSON is parsed and reduced to:

✓ RSpec: 42 passed (0.12s)

Or on failures:

RSpec: 38 passed, 2 failed, 3 pending (0.34s)
═══════════════════════════════════════

Failures:
1. ❌ User saves to database
   ./spec/models/user_spec.rb:10
   ExpectationNotMetError: expected true but got false
   ./spec/models/user_spec.rb:11:in `block'

2. ❌ Post sends notification
   ./spec/mailers/post_mailer_spec.rb:22
   ...

Key details:

• Auto-detects bundle exec rspec when a Gemfile mentioning rspec is present
• bin/rspec supported: Rails binstub pattern intercepted by hook
• Gem backtrace stripped: lines from /gems/ and lib/rspec are skipped
• Exception class shortened: RSpec::Expectations::ExpectationNotMetError → ExpectationNotMetError
• Cap at 5 failures inline, ... +N more for the rest
• Fallback: plain-text summary parser when JSON is unavailable

Test plan

• 17 unit tests (all paths, token savings ≥60%, edge cases, fallback parser)
• cargo fmt --all --check clean
• cargo clippy --all-targets clean (0 new warnings)
• Full test suite: 395/395 passing
• Manual: tested on real Rails project (attensi-backend)

  $ rtk rspec spec/models/accountability_spec.rb
  ✓ RSpec: 1 passed (3.94s)
  
  $ rtk rspec spec/models/avatar_spec.rb
  ✓ RSpec: 6 passed (1.12s)
  

🤖 Generated with Claude Code

[pszymkowiak]

Thanks for the contribution! Could you rebase on master? The branch is 2 commits behind. We'll review once it's up to date.