feat: add built-in upgrade command for self-update (#898)

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

cli/src/main.rs

@@ -7,6 +7,7 @@ mod native;
 mod output;
 #[cfg(test)]
 mod test_utils;
+mod upgrade;
 mod validation;
 
 use serde_json::json;
@@ -26,6 +27,7 @@ use install::run_install;
 use output::{
     print_command_help, print_help, print_response_with_opts, print_version, OutputOptions,
 };
+use upgrade::run_upgrade;
 
 fn serialize_json_value(value: &serde_json::Value) -> String {
     serde_json::to_string(value).unwrap_or_else(|_| {
@@ -226,6 +228,12 @@ fn main() {
         return;
     }
 
+    // Handle upgrade separately
+    if clean.first().map(|s| s.as_str()) == Some("upgrade") {
+        run_upgrade();
+        return;
+    }
+
     // Handle session separately (doesn't need daemon)
     if clean.first().map(|s| s.as_str()) == Some("session") {
         run_session(&clean, &flags.session, flags.json);