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/project-structure.md

@@ -0,0 +1,82 @@
+# Project Structure
+
+## Python
+
+Use src layout for all projects. Place application code in `src/<project>/`, tests in `tests/`.
+
+```
+project/
+├── src/myproject/
+│   ├── __init__.py
+│   ├── main.py          # Entry point
+│   └── core/
+│       └── module.py
+├── tests/
+│   ├── __init__.py
+│   └── test_module.py
+├── pyproject.toml       # Config
+├── README.md
+└── .gitignore
+```
+
+**Rules:**
+- One module per directory file
+- `__init__.py` in every package
+- Entry point in `src/myproject/main.py`
+- Config in root: `pyproject.toml`, `requirements.txt`
+
+## TypeScript
+
+Use `src/` for source, `dist/` for build output.
+
+```
+project/
+├── src/
+│   ├── index.ts         # Entry point
+│   ├── core/
+│   │   └── module.ts
+│   └── types.ts
+├── tests/
+│   └── module.test.ts
+├── package.json         # Config
+├── tsconfig.json
+└── README.md
+```
+
+**Rules:**
+- One module per file
+- Index exports from `src/index.ts`
+- Entry point in `src/index.ts`
+- Config in root: `package.json`, `tsconfig.json`
+
+## Nix
+
+Use `modules/` for NixOS modules, `pkgs/` for packages.
+
+```
+nix-config/
+├── modules/
+│   ├── default.nix      # Module list
+│   └── my-service.nix
+├── pkgs/
+│   └── my-package/
+│       └── default.nix
+├── flake.nix            # Entry point
+├── flake.lock
+└── README.md
+```
+
+**Rules:**
+- One module per file in `modules/`
+- One package per directory in `pkgs/`
+- Entry point in `flake.nix`
+- Config in root: `flake.nix`, shell.nix
+
+## General
+
+- Use hyphen-case for directories
+- Use kebab-case for file names
+- Config files in project root
+- Tests separate from source
+- Docs in root: README.md, CHANGELOG.md
+- Hidden configs: .env, .gitignore