fix(agents): honor COPILOT_HOME for Copilot hooks and MCP

[Fluzko]

What does this PR do?

Part of #277.
Depends on #346 and #333 (this branch carries #333's commit; until they merge, only the last commit is new here).

The Copilot CLI reads its settings and MCP servers from $COPILOT_HOME when that variable is set and ignores ~/.copilot/. Symposium still wrote its global hooks and MCP servers under ~/.copilot/, so for anyone who relocated their Copilot config they never loaded, while the plugins from #346 already went to $COPILOT_HOME.

Hooks and MCP servers now go to $COPILOT_HOME too. Without the variable nothing changes. A settings.json that is not a JSON object, or whose hooks is not one, no longer makes symposium panic: it reports an error and leaves the file untouched.

Verified against Copilot CLI 1.0.89 with COPILOT_HOME set: symposium's MCP server is listed and started, and every event runs symposium's hook.

Disclosure questions

AI disclosure.

• The AI tool authored large parts of the code

[Fluzko]

Folded into #333.