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/languages/shell.md

@@ -0,0 +1,100 @@
+# Shell Scripting Rules
+
+## Shebang
+
+Always use `#!/usr/bin/env bash` for portability. Never hardcode `/bin/bash`.
+
+```bash
+#!/usr/bin/env bash
+```
+
+## Strict Mode
+
+Enable strict mode in every script.
+
+```bash
+#!/usr/bin/env bash
+set -euo pipefail
+```
+
+- `-e`: Exit on error
+- `-u`: Error on unset variables
+- `-o pipefail`: Return exit status of last failed pipe command
+
+## Shellcheck
+
+Run shellcheck on all scripts before committing.
+
+```bash
+shellcheck script.sh
+```
+
+## Quoting
+
+Quote all variable expansions and command substitutions. Use arrays instead of word-splitting strings.
+
+```bash
+# Good
+"${var}"
+files=("file1.txt" "file2.txt")
+for f in "${files[@]}"; do
+  process "$f"
+done
+
+# Bad
+$var
+files="file1.txt file2.txt"
+for f in $files; do
+  process $f
+done
+```
+
+## Functions
+
+Define with parentheses, use `local` for variables.
+
+```bash
+my_function() {
+  local result
+  result=$(some_command)
+  echo "$result"
+}
+```
+
+## Command Substitution
+
+Use `$()` not backticks. Nests cleanly.
+
+```bash
+# Good
+output=$(ls "$dir")
+
+# Bad
+output=`ls $dir`
+```
+
+## POSIX Portability
+
+Write POSIX-compliant scripts when targeting `/bin/sh`.
+
+- Use `[[` only for bash scripts
+- Use `printf` instead of `echo -e`
+- Avoid `[[`, `((`, `&>` in sh scripts
+
+## Error Handling
+
+Use `trap` for cleanup.
+
+```bash
+cleanup() {
+  rm -f /tmp/lockfile
+}
+trap cleanup EXIT
+```
+
+## Readability
+
+- Use 2-space indentation
+- Limit lines to 80 characters
+- Add comments for non-obvious logic
+- Separate sections with blank lines