Extension system: Add priority-based resolution ordering

[mbachorik]

Summary

Port the priority-based resolution system from presets to extensions, allowing deterministic override ordering when multiple extensions provide similar commands.

Context

The preset system has PresetRegistry.list_by_priority() (presets.py:293-305) which returns presets sorted by priority for resolution. Extensions lack this capability.

Requirements

• Add priority field to ExtensionRegistry
• Implement list_by_priority() method in ExtensionRegistry
• Add --priority option to extension add command
• Apply priority ordering in command resolution
• Preserve priority on updates

Related

• Part of extension system parity with preset system
• See Feature Request: Auto-register extensions as Agent Skills #1795 for skills propagation (separate effort)