feat: add provider-declared features system with Codex fast mode (#186)

Introduce a generic feature system where providers declare dynamic
features (toggles/selects) and the app renders controls automatically.
One message pair (set_agent_feature_request/response) handles all
feature mutations. Feature values persist and restore on agent resume.

First consumer: Codex fast mode (service_tier) — gated to supported
model families, with proper cleanup on model switch.

packages/app/src/utils/agent-snapshots.ts

@@ -47,6 +47,7 @@ export function normalizeAgentSnapshot(snapshot: AgentSnapshotPayload, serverId:
     title: snapshot.title ?? null,
     cwd: snapshot.cwd,
     model: snapshot.model ?? null,
+    features: snapshot.features,
     thinkingOptionId: snapshot.thinkingOptionId ?? null,
     requiresAttention: snapshot.requiresAttention ?? false,
     attentionReason: snapshot.attentionReason ?? null,