feat: Add plugin system for gdlint

[naxa1ka]

Summary

Adds a plugin system to gdlint that allows loading external lint rules from Python packages.

Motivation

Currently, adding custom lint checks requires forking the toolkit. This makes it hard to maintain project-specific rules (e.g. async naming conventions, custom annotations) alongside upstream updates.

With this change, teams can distribute custom lint rules as pip-installable Python packages and load them via the plugins config key — no fork needed.

How it works

Add a plugins key to your gdlintrc with a list of Python module paths:

[gdlint]
plugins=my_custom_checks,another_plugin.rules

Each plugin is a Python module that exposes a lint(parse_tree, config) function:

from typing import List
from gdtoolkit.linter.problem import Problem

def lint(parse_tree, config) -> List[Problem]:
    problems = []
    # inspect parse_tree, append Problem instances
    return problems

Plugins are loaded via importlib.import_module() at lint time. Missing or failing plugins are logged as warnings and skipped — they never crash the linter.

Changes

• gdtoolkit/linter/__init__.py — added importlib/logging imports, "plugins": [] config key, and plugin loading loop
• tests/linter/test_plugins.py — 4 tests covering valid/missing/empty/no-plugins-key scenarios
• tests/linter/common.py — added multiple_nok_check test helper for multi-error assertions
• README.md — documented plugin usage under the Linter section
• .gitignore — added .venv/, .idea/, .claude/

Test results

All existing tests pass with no regressions:

1087 passed, 343 skipped in 14.37s