feat(rules): add initial rule files for concerns, languages, and frameworks

Concerns (6 files):
- coding-style.md (163 lines): patterns, anti-patterns, error handling, SOLID
- naming.md (105 lines): naming conventions table per language
- documentation.md (149 lines): docstrings, WHY vs WHAT, README standards
- testing.md (134 lines): AAA pattern, mocking philosophy, TDD
- git-workflow.md (118 lines): conventional commits, branch naming, PR format
- project-structure.md (82 lines): directory layout, entry points, config placement

Languages (4 files):
- python.md (224 lines): uv, ruff, pyright, pytest, pydantic, idioms, anti-patterns
- typescript.md (150 lines): strict mode, discriminated unions, satisfies, as const
- nix.md (129 lines): flake structure, module patterns, alejandra, anti-patterns
- shell.md (100 lines): set -euo pipefail, shellcheck, quoting, POSIX

Frameworks (1 file):
- n8n.md (42 lines): workflow design, node patterns, Error Trigger, security

Context budget: 975 lines (concerns + python) < 1500 limit

Refs: T6-T16 of rules-system plan

rules/languages/python.md

@@ -0,0 +1,224 @@
+# Python Language Rules
+
+## Toolchain
+
+### Package Management (uv)
+```bash
+uv init my-project --package
+uv add numpy pandas
+uv add --dev pytest ruff pyright hypothesis
+uv run python -m pytest
+uv lock --upgrade-package numpy
+```
+
+### Linting & Formatting (ruff)
+```toml
+[tool.ruff]
+line-length = 100
+target-version = "py311"
+
+[tool.ruff.lint]
+select = ["E", "F", "W", "I", "N", "UP"]
+ignore = ["E501"]
+
+[tool.ruff.format]
+quote-style = "double"
+```
+
+### Type Checking (pyright)
+```toml
+[tool.pyright]
+typeCheckingMode = "strict"
+reportMissingTypeStubs = true
+reportUnknownMemberType = true
+```
+
+### Testing (pytest + hypothesis)
+```python
+import pytest
+from hypothesis import given, strategies as st
+
+@given(st.integers(), st.integers())
+def test_addition_commutative(a, b):
+    assert a + b == b + a
+
+@pytest.fixture
+def user_data():
+    return {"name": "Alice", "age": 30}
+
+def test_user_creation(user_data):
+    user = User(**user_data)
+    assert user.name == "Alice"
+```
+
+### Data Validation (Pydantic)
+```python
+from pydantic import BaseModel, Field, validator
+
+class User(BaseModel):
+    name: str = Field(min_length=1, max_length=100)
+    age: int = Field(ge=0, le=150)
+    email: str
+
+    @validator('email')
+    def email_must_contain_at(cls, v):
+        if '@' not in v:
+            raise ValueError('must contain @')
+        return v
+```
+
+## Idioms
+
+### Comprehensions
+```python
+# List comprehension
+squares = [x**2 for x in range(10) if x % 2 == 0]
+
+# Dict comprehension
+word_counts = {word: text.count(word) for word in unique_words}
+
+# Set comprehension
+unique_chars = {char for char in text if char.isalpha()}
+```
+
+### Context Managers
+```python
+# Built-in context managers
+with open('file.txt', 'r') as f:
+    content = f.read()
+
+# Custom context manager
+from contextlib import contextmanager
+
+@contextmanager
+def timer():
+    start = time.time()
+    yield
+    print(f"Elapsed: {time.time() - start:.2f}s")
+```
+
+### Generators
+```python
+def fibonacci():
+    a, b = 0, 1
+    while True:
+        yield a
+        a, b = b, a + b
+
+def read_lines(file_path):
+    with open(file_path) as f:
+        for line in f:
+            yield line.strip()
+```
+
+### F-strings
+```python
+name = "Alice"
+age = 30
+
+# Basic interpolation
+msg = f"Name: {name}, Age: {age}"
+
+# Expression evaluation
+msg = f"Next year: {age + 1}"
+
+# Format specs
+msg = f"Price: ${price:.2f}"
+msg = f"Hex: {0xFF:X}"
+```
+
+## Anti-Patterns
+
+### Bare Except
+```python
+# AVOID: Catches all exceptions including SystemExit
+try:
+    risky_operation()
+except:
+    pass
+
+# USE: Catch specific exceptions
+try:
+    risky_operation()
+except ValueError as e:
+    log_error(e)
+except KeyError as e:
+    log_error(e)
+```
+
+### Mutable Defaults
+```python
+# AVOID: Default argument created once
+def append_item(item, items=[]):
+    items.append(item)
+    return items
+
+# USE: None as sentinel
+def append_item(item, items=None):
+    if items is None:
+        items = []
+    items.append(item)
+    return items
+```
+
+### Global State
+```python
+# AVOID: Global mutable state
+counter = 0
+
+def increment():
+    global counter
+    counter += 1
+
+# USE: Class-based state
+class Counter:
+    def __init__(self):
+        self.count = 0
+
+    def increment(self):
+        self.count += 1
+```
+
+### Star Imports
+```python
+# AVOID: Pollutes namespace, unclear origins
+from module import *
+
+# USE: Explicit imports
+from module import specific_function, MyClass
+import module as m
+```
+
+## Project Setup
+
+### pyproject.toml Structure
+```toml
+[project]
+name = "my-project"
+version = "0.1.0"
+requires-python = ">=3.11"
+dependencies = [
+    "pydantic>=2.0",
+    "httpx>=0.25",
+]
+
+[project.optional-dependencies]
+dev = ["pytest", "ruff", "pyright", "hypothesis"]
+
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+```
+
+### src Layout
+```
+my-project/
+├── pyproject.toml
+└── src/
+    └── my_project/
+        ├── __init__.py
+        ├── main.py
+        └── utils/
+            ├── __init__.py
+            └── helpers.py
+```