Extension system: Add template type diversity (scripts support)

[mbachorik]

Summary

Extend extensions to support multiple template types beyond commands, specifically adding scripts support.

Context

Presets support 3 template types (presets.py:122-127):

• template (e.g., spec-template)
• command (for agent overrides)
• script (shell scripts with .sh or .ps1)

Extensions only support command type currently.

Requirements

• Extend ExtensionManifest to accept provides.scripts array
• Add script file validation (path safety, format)
• Implement script registration/installation in ExtensionManager
• Update resolver to handle script lookups

Related

• Part of extension system parity with preset system