feat: improve installation system with user-local and system-wide options

Update justfile installation system to provide better user experience:

Installation Improvements:
- `just install` - installs to ~/bin (user-local, no sudo required)
- `just system-install` - installs to /usr/local/bin (system-wide, requires sudo)
- `just uninstall` - removes from ~/bin
- `just system-uninstall` - removes from /usr/local/bin (requires sudo)

Benefits:
- User-local installation by default (follows Unix best practices)
- No sudo required for personal installations
- Clear separation between user and system installs
- Easy uninstallation for both scenarios
- Helpful PATH reminder for ~/bin installation

Documentation Updates:
- Update CLAUDE.md with new installation commands
- Update IMPLEMENTATION_COMPARISON.md deployment examples
- Include uninstall instructions for both methods

This follows modern software distribution practices where user-local
installation is preferred for development tools, with system-wide
installation available when needed for shared environments.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

IMPLEMENTATION_COMPARISON.md

@@ -493,13 +493,13 @@ export MAIL2COUCH_LOG_FORMAT=json
 ### Universal Installation
 
 ```bash
-# Build and install both implementations
-just build-release
-sudo just install
+# Build and install both implementations (user-local)
+just install
+# This installs to ~/bin/mail2couch-go and ~/bin/mail2couch-rs
 
-# This installs:
-# - /usr/local/bin/mail2couch-go
-# - /usr/local/bin/mail2couch-rs
+# Build and install both implementations (system-wide)
+sudo just system-install
+# This installs to /usr/local/bin/mail2couch-go and /usr/local/bin/mail2couch-rs
 ```
 
 ---