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/tools/session-manager/tools.ts

@@ -70,7 +70,7 @@ export function createSessionManagerTools(ctx: PluginInput): Record<string, Tool
     },
     execute: async (args: SessionReadArgs, _context) => {
       try {
-        if (!sessionExists(args.session_id)) {
+        if (!(await sessionExists(args.session_id))) {
           return `Session not found: ${args.session_id}`
         }