Adds a new `agent-browser upgrade` command that automatically detects the installation method (npm, Homebrew, or Cargo) and runs the appropriate update command.
**Changes:**
- Added new `upgrade.rs` module with upgrade logic
- Updated `main.rs` to handle the `upgrade` command
- Added upgrade help text in `output.rs`
- Updated README.md and documentation with upgrade instructions
- Updated SKILL.md to mention the upgrade command
**Implementation details:**
- Fetches latest version from npm registry to show version diff
- Auto-detects installation method by checking Homebrew, Cargo paths, and npm global packages
- Provides fallback instructions if installation method cannot be determined
- Uses existing color module for consistent styled output
- Gracefully handles network failures and continues with upgrade
Fixes#895
Chris Tate
8cfba1752d