Hook messages during session start/resume can now be deferred and released later

What

• The main session controller gained deferHookMessages/releasePendingHookMessages methods, wired into the REPL as deferHookMessages/releaseDeferredHookMessages props, letting a pending hook set's transcript messages be held back instead of inserted immediately, until explicitly released.
• Before running a new turn (when starting fresh rather than resuming a concurrent query), the agent now awaits any pending/deferred hook set via awaitPendingHooks(), so the turn doesn't start until hooks have resolved (the wait is skipped only if the abort signal actually fires).
• On session resume, the mechanism for landing SessionStart-type hook messages into the transcript was reworked: instead of a single pending-promise plus a "landed" flag, there is now a _pendingHookSet object with deferHookMessages, awaitStartupHooks, releasePendingHookMessages, and abort support, tied to session id checks so stale hook results for a since-replaced session get discarded.

Why This makes hook-driven transcript messages around session start and resume more reliable: turns wait for pending hooks instead of racing them, hook messages can be staged and released at the right moment, and results from a stale, already-replaced session no longer leak into the current one.