feat(agent-plugins): install global plugins for Copilot CLI

[Fluzko]

What does this PR do?

Part of #277.
Depends on #332 (until it merges, only the last 2 commits are new here).
Needed by #334.

A plugin enabled with cargo agents use --global still reached the Copilot CLI only as per-project copies of its skills, although Copilot loads plugins from a local marketplace.

Symposium now registers ~/.symposium/installed/ as the marketplace symposium in ~/.copilot/settings.json (or $COPILOT_HOME/settings.json) and enables each plugin from it, the same two entries copilot plugin install writes. Copilot loads the plugin in place in every project, so an edit reaches the next session. Symposium touches only its own entries and keeps a plugin you turned off in Copilot; a settings file it cannot read is left untouched and the skills arrive per project as before.

Verified with Copilot CLI 1.0.89 through cargo agents sync, by asking the agent what it loaded.

Take into account:

• A plugin installed by the session-start sync is offered after /restart or in the next session.

Disclosure questions

AI disclosure.

• The AI tool authored large parts of the code