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/frameworks/n8n.md

@@ -0,0 +1,42 @@
+# n8n Workflow Automation Rules
+
+## Workflow Design
+- Start with a clear trigger: Webhook, Schedule, or Event source
+- Keep workflows under 20 nodes for maintainability
+- Group related logic with sub-workflows
+- Use the "Switch" node for conditional branching
+- Add "Wait" nodes between rate-limited API calls
+
+## Node Naming
+- Use verb-based names: `Fetch Users`, `Transform Data`, `Send Email`
+- Prefix data nodes: `Get_`, `Set_`, `Update_`
+- Prefix conditionals: `Check_`, `If_`, `When_`
+- Prefix actions: `Send_`, `Create_`, `Delete_`
+- Add version suffix to API nodes: `API_v1_Users`
+
+## Error Handling
+- Always add an Error Trigger node
+- Route errors to a "Notify Failure" branch
+- Log error details: `$json.error.message`, `$json.node.name`
+- Send alerts on critical failures
+- Add "Continue On Fail" for non-essential nodes
+
+## Data Flow
+- Use "Set" nodes to normalize output structure
+- Reference previous nodes: `{{ $json.field }}`
+- Use "Merge" node to combine multiple data sources
+- Apply "Code" node for complex transformations
+- Clean data before sending to external APIs
+
+## Credential Security
+- Store all secrets in n8n credentials manager
+- Never hardcode API keys or tokens
+- Use environment-specific credential sets
+- Rotate credentials regularly
+- Limit credential scope to minimum required permissions
+
+## Testing
+- Test each node independently with "Execute Node"
+- Verify data structure at each step
+- Mock external dependencies during development
+- Log workflow execution for debugging