fix: make sessionExists() async with SDK verification on SQLite

sessionExists() previously returned unconditional true on SQLite,
preventing ralph-loop orphaned-session cleanup from triggering.
Now uses sdkClient.session.messages() to verify session actually
exists. Callers updated to await the async result.

Addresses Cubic review feedback on PR #1837.

src/plugin/hooks/create-session-hooks.ts

@@ -122,7 +122,7 @@ export function createSessionHooks(args: {
     ? safeHook("ralph-loop", () =>
         createRalphLoopHook(ctx, {
           config: pluginConfig.ralph_loop,
-          checkSessionExists: async (sessionId) => sessionExists(sessionId),
+          checkSessionExists: async (sessionId) => await sessionExists(sessionId),
         }))
     : null