docs: clean up project documentation

- Convert all headings from underline style to ATX style
- Fix heading hierarchy in INSTALLING.md (H3→H2, H4→H3)
- Move Installing section before Usage in README
- Bump install snippet version from 4.5.1 to 4.7.0
- Add feature request guidance to CONTRIBUTING.md
- Add installer package and .cheat discovery to CLAUDE.md
- Remove unused reference links and dead HTML comments
- Trim trailing whitespace

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -85,7 +85,11 @@ The `cheat` command-line tool is organized into several key packages:
    - Writes to stdout or pager
    - Handles text formatting and indentation
 
-6. **`internal/repo`**: Git repository management
+6. **`internal/installer`**: First-run installer
+   - Prompts user for initial configuration choices
+   - Generates default `conf.yml` and downloads community cheatsheets
+
+7. **`internal/repo`**: Git repository management
    - Clones community cheatsheet repositories
    - Updates existing repositories
 
@@ -95,6 +99,7 @@ The `cheat` command-line tool is organized into several key packages:
 - **Override mechanism**: Local sheets override community sheets with same name
 - **Tag system**: Sheets can be categorized with tags in frontmatter
 - **Multiple cheatpaths**: Supports personal, community, and directory-scoped sheets
+- **Directory-scoped discovery**: Walks up from cwd to find the nearest `.cheat` directory (like `.git` discovery)
 
 ### Sheet Format