Extension system: Add source attribution tracking

[mbachorik]

Summary

Track and expose which extension provided each command/template for better debugging and transparency.

Context

Presets have resolve_with_source() (presets.py:1477-1530) which returns source attribution showing which preset/extension/core supplied a template.

Extensions lack this capability.

Requirements

• Track source extension in registry for each registered command
• Add resolve_with_source() method to ExtensionResolver
• Expose source info in extension info command
• Support debugging override chains

Related

• Depends on: template resolution system
• Part of extension system parity with preset system