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/concerns/documentation.md

@@ -0,0 +1,149 @@
+# Documentation Rules
+
+## When to Document
+
+**Document public APIs**. Every public function, class, method, and module needs documentation. Users need to know how to use your code.
+**Document complex logic**. Algorithms, state machines, and non-obvious implementations need explanations. Future readers will thank you.
+**Document business rules**. Encode domain knowledge directly in comments. Don't make anyone reverse-engineer requirements from code.
+**Document trade-offs**. When you choose between alternatives, explain why. Help future maintainers understand the decision context.
+**Do NOT document obvious code**. Comments like `// get user` add noise. Delete them.
+
+## Docstring Formats
+
+### Python (Google Style)
+
+```python
+def calculate_price(quantity: int, unit_price: float, discount: float = 0.0) -> float:
+    """Calculate total price after discount.
+    Args:
+        quantity: Number of items ordered.
+        unit_price: Price per item in USD.
+        discount: Decimal discount rate (0.0 to 1.0).
+    Returns:
+        Final price in USD.
+    Raises:
+        ValueError: If quantity is negative.
+    """
+```
+
+### JavaScript/TypeScript (JSDoc)
+
+```javascript
+/**
+ * Validates user input against security rules.
+ * @param {string} input - Raw user input from form.
+ * @param {Object} rules - Validation constraints.
+ * @param {number} rules.maxLength - Maximum allowed length.
+ * @returns {boolean} True if input passes all rules.
+ * @throws {ValidationError} If input violates security constraints.
+ */
+function validateInput(input, rules) {
+```
+
+### Bash
+
+```bash
+#!/usr/bin/env bash
+# Deploy application to production environment.
+#
+# Usage: ./deploy.sh [environment]
+#
+# Args:
+#   environment: Target environment (staging|production). Default: staging.
+#
+# Exits:
+#   0 on success, 1 on deployment failure.
+```
+
+## Inline Comments: WHY Not WHAT
+
+**Incorrect:**
+```python
+# Iterate through all users
+for user in users:
+    # Check if user is active
+    if user.active:
+        # Increment counter
+        count += 1
+```
+
+**Correct:**
+```python
+# Count only active users to calculate monthly revenue
+for user in users:
+    if user.active:
+        count += 1
+```
+
+**Incorrect:**
+```javascript
+// Set timeout to 5000
+setTimeout(() => {
+  // Show error message
+  alert('Error');
+}, 5000);
+```
+
+**Correct:**
+```javascript
+// 5000ms delay prevents duplicate alerts during rapid retries
+setTimeout(() => {
+  alert('Error');
+}, 5000);
+```
+
+**Incorrect:**
+```bash
+# Remove temporary files
+rm -rf /tmp/app/*
+```
+
+**Correct:**
+```bash
+# Clear temp directory before batch import to prevent partial state
+rm -rf /tmp/app/*
+```
+
+**Rule:** Describe the intent and context. Never describe what the code obviously does.
+
+## README Standards
+
+Every project needs a README at the top level.
+
+**Required sections:**
+1. **What it does** - One sentence summary
+2. **Installation** - Setup commands
+3. **Usage** - Basic example
+4. **Configuration** - Environment variables and settings
+5. **Contributing** - How to contribute
+
+**Example structure:**
+
+```markdown
+# Project Name
+
+One-line description of what this project does.
+
+## Installation
+```bash
+npm install
+```
+
+## Usage
+```bash
+npm start
+```
+
+## Configuration
+
+Create `.env` file:
+```
+API_KEY=your_key_here
+```
+
+## Contributing
+
+See [CONTRIBUTING.md](./CONTRIBUTING.md).
+```
+
+**Keep READMEs focused**. Link to separate docs for complex topics. Don't make the README a tutorial.