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/naming.md

@@ -0,0 +1,105 @@
+# Naming Conventions
+
+Use consistent naming across all code. Follow language-specific conventions.
+
+## Language Reference
+
+| Type | Python | TypeScript | Nix | Shell |
+|------|--------|------------|-----|-------|
+| Variables | snake_case | camelCase | camelCase | UPPER_SNAKE |
+| Functions | snake_case | camelCase | camelCase | lower_case |
+| Classes | PascalCase | PascalCase | - | - |
+| Constants | UPPER_SNAKE | UPPER_SNAKE | camelCase | UPPER_SNAKE |
+| Files | snake_case | camelCase | hyphen-case | hyphen-case |
+| Modules | snake_case | camelCase | - | - |
+
+## General Rules
+
+**Files**: Use hyphen-case for documentation, snake_case for Python, camelCase for TypeScript. Names should describe content.
+
+**Variables**: Use descriptive names. Avoid single letters except loop counters. No Hungarian notation.
+
+**Functions**: Use verb-noun pattern. Name describes what it does, not how it does it.
+
+**Classes**: Use PascalCase with descriptive nouns. Avoid abbreviations.
+
+**Constants**: Use UPPER_SNAKE with descriptive names. Group related constants.
+
+## Examples
+
+Python:
+```python
+# Variables
+user_name = "alice"
+is_authenticated = True
+
+# Functions
+def get_user_data(user_id):
+    pass
+
+# Classes
+class UserProfile:
+    pass
+
+# Constants
+MAX_RETRIES = 3
+API_ENDPOINT = "https://api.example.com"
+```
+
+TypeScript:
+```typescript
+// Variables
+const userName = "alice";
+const isAuthenticated = true;
+
+// Functions
+function getUserData(userId: string): User {
+    return null;
+}
+
+// Classes
+class UserProfile {
+    private name: string;
+}
+
+// Constants
+const MAX_RETRIES = 3;
+const API_ENDPOINT = "https://api.example.com";
+```
+
+Nix:
+```nix
+# Variables
+let
+  userName = "alice";
+  isAuthenticated = true;
+in
+# ...
+```
+
+Shell:
+```bash
+# Variables
+USER_NAME="alice"
+IS_AUTHENTICATED=true
+
+# Functions
+get_user_data() {
+    echo "Getting data"
+}
+
+# Constants
+MAX_RETRIES=3
+API_ENDPOINT="https://api.example.com"
+```
+
+## File Naming
+
+Use these patterns consistently. No exceptions.
+
+- Skills: `hyphen-case`
+- Python: `snake_case.py`
+- TypeScript: `camelCase.ts` or `hyphen-case.ts`
+- Nix: `hyphen-case.nix`
+- Shell: `hyphen-case.sh`
+- Markdown: `UPPERCASE.md` or `sentence-case.md`