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/git-workflow.md

@@ -0,0 +1,118 @@
+# Git Workflow Rules
+
+## Conventional Commits
+
+Format: `<type>(<scope>): <subject>`
+
+### Commit Types
+
+- **feat**: New feature
+  - `feat(auth): add OAuth2 login flow`
+  - `feat(api): expose user endpoints`
+
+- **fix**: Bug fix
+  - `fix(payment): resolve timeout on Stripe calls`
+  - `fix(ui): button not clickable on mobile`
+
+- **refactor**: Code refactoring (no behavior change)
+  - `refactor(utils): extract date helpers`
+  - `refactor(api): simplify error handling`
+
+- **docs**: Documentation only
+  - `docs(readme): update installation steps`
+  - `docs(api): add endpoint examples`
+
+- **chore**: Maintenance tasks
+  - `chore(deps): update Node to 20`
+  - `chore(ci): add GitHub actions workflow`
+
+- **test**: Tests only
+  - `test(auth): add unit tests for login`
+  - `test(e2e): add checkout flow tests`
+
+- **style**: Formatting, no logic change
+  - `style: sort imports alphabetically`
+
+### Commit Rules
+
+- Subject max 72 chars
+- Imperative mood ("add", not "added")
+- No period at end
+- Reference issues: `Closes #123`
+
+## Branch Naming
+
+Pattern: `<type>/<short-description>`
+
+### Branch Types
+
+- `feature/add-user-dashboard`
+- `feature/enable-dark-mode`
+- `fix/login-redirect-loop`
+- `fix/payment-timeout-error`
+- `refactor/extract-user-service`
+- `refactor/simplify-auth-flow`
+- `hotfix/security-vulnerability`
+
+### Branch Rules
+
+- Lowercase and hyphens
+- Max 50 chars
+- Delete after merge
+
+## Pull Requests
+
+### PR Title
+
+Follow Conventional Commit format:
+- `feat: add user dashboard`
+- `fix: resolve login redirect loop`
+
+### PR Description
+
+```markdown
+## What
+Brief description
+
+## Why
+Reason for change
+
+## How
+Implementation approach
+
+## Testing
+Steps performed
+
+## Checklist
+- [ ] Tests pass
+- [ ] Code reviewed
+- [ ] Docs updated
+```
+
+## Merge Strategy
+
+### Squash Merge
+
+- Many small commits
+- One cohesive feature
+- Clean history
+
+### Merge Commit
+
+- Preserve commit history
+- Distinct milestones
+- Detailed history preferred
+
+### When to Rebase
+
+- Before opening PR
+- Resolving conflicts
+- Keeping current with main
+
+## General Rules
+
+- Pull latest from main before starting
+- Write atomic commits
+- Run tests before pushing
+- Request peer review before merge
+- Never force push to main/master