Extension system: Add template resolution system

[mbachorik]

Summary

Create an ExtensionResolver class similar to PresetResolver, making extension templates discoverable and resolvable alongside preset templates.

Context

The preset system has a 4-tier resolution stack (presets.py:1374-1530):

1. Project-local overrides
2. Installed presets (by priority)
3. Extension-provided templates
4. Core templates

Extensions are already integrated at line 1449, but have no standalone resolver or resolve_with_source() capability.

Requirements

• Create ExtensionResolver class mirroring PresetResolver
• Implement resolution with extension templates included
• Add source attribution via resolve_with_source()
• Support template listing and discovery

Related

• Depends on: priority-based resolution
• Part of extension system parity with preset system