docs: add CLAUDE.md with codebase guide

CLAUDE.md

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+# CLAUDE.md
+
+## Project Overview
+
+Apache File Directory is a static HTML page that replicates the Apache HTTP Server's default file directory listing. The entire application is a single `index.html` file with supporting GIF icons. It uses intentionally retro HTML 3.2 markup for Apache compatibility.
+
+## Repository Structure
+
+```
+index.html          # Main application file (the directory listing template)
+icons/              # GIF icons for directory entry types (folder, text, etc.)
+docs/               # User-facing documentation (usage, customization, legal)
+.github/workflows/  # CI: Docker publish, GitHub Pages deploy, Gitleaks secret scanning
+.github/agents/     # GitHub agent definitions (lint, test, docs)
+Dockerfile          # Nginx-based container for serving the page
+```
+
+## Development
+
+**No build step, no dependencies, no automated tests.** This is a single static HTML file.
+
+- **Run locally:** Open `index.html` in a browser, or use Docker:
+  ```
+  docker build -t afd . && docker run -d -p 8000:80 afd
+  ```
+- **Test:** Manual browser testing. Validate HTML with the W3C Markup Validator.
+- **Lint:** Use Prettier for formatting. DeepSource runs static analysis via `.deepsource.toml`.
+
+## Code Conventions
+
+- **Indentation:** 4 spaces (no tabs)
+- **HTML standard:** W3C HTML 3.2 Final DOCTYPE — intentionally retro, do not modernize
+- **Comments:** Comment all functions and code sections
+- **Commit messages:** Conventional Commits format (`feat:`, `fix:`, `docs:`, `build(deps):`)
+- **Versioning:** Semantic Versioning (current: 1.3.0, tracked in `package.json` and `CHANGELOG.md`)
+- **License:** Apache License 2.0 — preserve copyright headers
+
+## CI/CD
+
+- **GitHub Pages:** Auto-deploys on push to `main`
+- **Docker:** Builds and pushes to `ghcr.io/willtheorangeguy/apache-file-directory` on push to `main`
+- **Gitleaks:** Scans for secrets on PRs and pushes
+- **Dependabot:** Daily updates for GitHub Actions and Docker base images
+
+## Key Files
+
+| File                    | Purpose                                               |
+| ----------------------- | ----------------------------------------------------- |
+| `index.html`            | The entire application                                |
+| `Dockerfile`            | Nginx container serving static files                  |
+| `package.json`          | Project metadata only (no runtime dependencies)       |
+| `CONTRIBUTING.md`       | Contribution guidelines (GitHub Flow, 4-space indent) |
+| `CHANGELOG.md`          | Version history                                       |
+| `docs/CUSTOMIZATION.md` | Guide for personalizing the template                  |