feat: add layered AI rules system (.claude/rules/)

[buddingengineers12345]

Summary

Adds a layered plain-Markdown rules system under .claude/rules/ (template maintainer context) and template/.claude/rules/ (generated project context). Rules are tool-agnostic — they work with Claude Code, Cursor, or any LLM that can read context from a directory.

What was added

Root .claude/rules/ (template maintainer)

.claude/rules/
├── README.md              ← structure overview + how to write new rules
├── common/                ← language-agnostic principles
│   ├── coding-style.md
│   ├── git-workflow.md
│   ├── testing.md
│   ├── security.md
│   ├── development-workflow.md
│   └── code-review.md
├── python/                ← Python-specific (extends common/)
│   ├── coding-style.md
│   ├── testing.md
│   ├── patterns.md
│   ├── security.md
│   └── hooks.md
├── jinja/                 ← Jinja2 templates
│   ├── coding-style.md
│   └── testing.md
├── bash/                  ← shell hook scripts
│   ├── coding-style.md
│   └── security.md
├── markdown/
│   └── conventions.md
├── yaml/
│   └── conventions.md
└── copier/                ← Copier template conventions (this repo only)
    └── template-conventions.md

template/.claude/rules/ (generated projects)

Subset covering common, python, bash, and markdown — no Jinja or Copier rules (generated projects do not use those):

• common/: coding-style, git-workflow, testing, security, development-workflow, code-review
• python/: coding-style (Jinja — references {{ package_name }}), testing, patterns (Jinja), security, hooks
• bash/: coding-style, security
• markdown/: conventions

Two Python files use the .jinja suffix because they reference {{ package_name }} from Copier variables.

CLAUDE.md and template/CLAUDE.md.jinja

Added an "AI rules" section pointing to the rules directory and describing the dual-hierarchy.

Design decisions

• Plain Markdown, no tool-specific syntax: no .mdc frontmatter, no Cursor-specific fields. Any tool that can read a file benefits from them.
• Layered common → language-specific: matches the pattern from the reference repo. Language-specific rules override common rules where they conflict.
• Dual hierarchy preserved: root rules for template maintainers; template rules for generated projects. Copier/Jinja/YAML rules stay in the root only.
• # applies-to: comments: optional YAML-comment annotations at the top of language-specific files indicate which glob patterns apply; tools that understand frontmatter can use them, others ignore them.
• README.md explains how to add new rules: covers directory choice, naming, the extending-common pattern, and the dual-hierarchy mirror requirement.