feat: add generic linter output filter (eslint/ruff/mypy/pylint)

[bug-ops]

Parent: #717

Problem

Linters like eslint, ruff, mypy, pylint produce per-file warnings that can be grouped by rule (like the existing Clippy filter).

Acceptance Criteria

• Match eslint, ruff check, mypy, pylint, flake8
• Group warnings by rule/code (reuse shared group_by_rule utility)
• Show count per rule, list affected files
• Pass through errors unchanged
• Config: [tools.filters.linter] with enabled flag
• Tests
• Expected savings: 60-80%