fix(plugin): normalize event session ids

Handle OpenCode session events that carry the session ID under properties.info.id or properties.info.sessionID so background tasks and continuation hooks do not miss idle/error/delete events.

Add regression coverage for nested session.idle events completing background tasks and waking continuation hooks.

src/shared/index.ts

@@ -54,6 +54,7 @@ export * from "./fallback-model-availability"
 export * from "./connected-providers-cache"
 export * from "./context-limit-resolver"
 export * from "./session-utils"
+export * from "./event-session-id"
 export * from "./tmux"
 export * from "./model-suggestion-retry"
 export * from "./opencode-server-auth"