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.

[FlorianBruniaux]

Thanks for the mypy support — this fits well in the Python tooling story alongside ruff/pytest/pip.

The PR has a merge conflict with master — could you rebase? A lot has changed since Feb 13 (Python/Go support landed in 0.18.0).

Once rebased, I'll do a proper review. The approach (grouped errors by file, top error codes summary) looks solid from the description.

[Rmohid]

Addressed all 4 review items:

1. Removed specs/ -- deleted all 5 process doc files (spec, plan, tasks, research, checklists)
2. Cleaned CLAUDE.md -- removed "Active Technologies" and "Recent Changes" sections; kept only the mypy module table entry
3. Deduplicated mypy handler -- made mypy_cmd::filter_mypy_output() public and lint_cmd.rs now delegates to it (removed 145 lines of duplicate code + 2 redundant tests). The mypy_cmd version is more capable: handles column numbers, note continuations, file-less errors, and shows full error details
4. Rebased on master (c29644b, v0.21.1)

All 387 tests pass, including 11 mypy-specific tests. Zero new clippy warnings.

[Rmohid]

@FlorianBruniaux All 4 review items have been addressed (see previous comment for details). Could you remove the awaiting-changes label and merge when ready? Thanks!