feat: add mypy command with grouped error output

[Rmohid]

Summary

• Add rtk mypy command that parses mypy type checker output, groups errors by file (sorted by error count), shows top error codes summary, and preserves note continuation lines (~80% token reduction)
• Add discovery registry pattern and hook rewrites for mypy and python -m mypy
• 11 new tests (324 total), all passing

Test plan

• All 9 mypy_cmd unit tests pass (grouped output, column numbers, top codes, single code, every error shown, note continuation, fileless errors, no errors, no file limit)
• Both registry tests pass (test_classify_mypy, test_classify_python_m_mypy)
• test_patterns_rules_length_match passes (PATTERNS/RULES aligned at 22)
• Full pre-commit gate: cargo fmt --all --check && cargo clippy --all-targets && cargo test

🤖 Generated with Claude Code

[pszymkowiak]

Rust code ok, 4 items to address:

1. Remove specs/ — 533 lines of process docs (spec, plan, tasks, research, checklists). Should not be in the repo.
2. Clean CLAUDE.md — remove the "Active Technologies" and "Recent Changes" sections added at the bottom. Keep only the module table entry.
3. Duplicate rtk lint mypy — already merged in feat(python): add lint dispatcher + universal format command #100. Either remove the duplicate parser in lint_cmd.rs or have it delegate to mypy_cmd.
4. Rebase needed — mergeStateStatus: UNSTABLE.