feat(fleet): export MOSAIC_AGENT_CLASS into agent env (A3a) (#663)

packages/mosaic/src/commands/fleet.ts

@@ -490,6 +490,9 @@ export function generateAgentEnv(roster: FleetRoster, agent: FleetAgent): string
   const workingDirectory = agent.workingDirectory ?? roster.defaults.workingDirectory;
   return [
     `MOSAIC_AGENT_NAME=${shellEnvValue(agent.name)}`,
+    // Per-agent class → start-agent-session.sh / launcher reads this to inject the
+    // matching persona contract for the agent's class (default `worker`).
+    `MOSAIC_AGENT_CLASS=${shellEnvValue(agent.className)}`,
     `MOSAIC_AGENT_RUNTIME=${shellEnvValue(agent.runtime)}`,
     // Per-agent model hint → start-agent-session.sh appends `--model <hint>` to
     // the `mosaic yolo` launch so workers run on the roster's model (e.g. pi on