Persist agent service state instead of keeping agents only in memory

### Task Summary

Each agent in the agent service is a stateful in-memory object with no durable backing store.

In `agent-service/src/server.ts` the entire fleet lives in a process-local map:

```
const agentStore = new Map();
let agentCounter = 0;
```

All agent state — the ReAct step tree, current HEAD/checkout pointer, per-agent settings, delegate config, and the cached operator-result state — is held in `TexeraAgent` instance fields. The only thing persisted today is the **workflow content**, which is pushed back to the dashboard backend (`persistWorkflow`, debounced 500ms). The agent and its conversation/step history are not.

Consequences:

```
Before: restart / crash / redeploy -> agentStore is empty -> all agents + history lost
second replica -> separate agentStore -> agent not found
After: restart / crash / redeploy -> agents rehydrated from store
any replica -> shared store -> agent reachable
```

This also blocks horizontal scaling: a request routed to a replica that doesn't hold the agent gets `Agent not found`.

Introduce a persistence layer (e.g. database or Redis) for agent records and their step history, with load-on-demand / rehydration on startup, so agents survive restarts and can be served by more than one replica.

### Task Type

- [ ] Refactor / Cleanup
- [ ] DevOps / Deployment / CI
- [ ] Testing / QA
- [ ] Documentation
- [ ] Performance
- [x] Other