fix: migrate extension commands on integration switch (#2404)

* fix: migrate extension commands on integration switch

When switching integrations (e.g. kimi → opencode), extension commands
were not re-registered for the new agent, leaving the new agent without
extension support and orphaning files in the old agent's directory.

Changes:
- Add ExtensionManager.unregister_agent_artifacts() to clean up old
  agent extension files and registry entries during switch
- Add ExtensionManager.register_enabled_extensions_for_agent() to
  re-register all enabled extensions for the new agent
- Wire both into integration_switch() after uninstall/install phases
- Handle skills mode (Copilot --skills) correctly
- Add tests for kimi→opencode→claude migration, Copilot skills mode,
  and disabled extension handling

Fixes extension commands not appearing after integration switch.

* Update src/specify_cli/extensions.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>