Agent follow-up questions with session memory

## Summary
Enable follow-up interactions that reuse prior agent session context.

## Feature goal
Allow `session_id` reuse in `/agent/run` and expose session history endpoint.

## Scope
- Persist memory per session ID.
- Add read endpoint for session history.

## Hints
- Expand `src/agent/memory.py` for session registry.
- Extend `src/api/agent.py` routes.

## Acceptance criteria
- Reusing `session_id` keeps prior context.
- New session ID starts clean state.
- History endpoint returns chronological steps.

## Test note
Run two calls with same session ID and assert second run sees prior context.

Start in src/agent/memory.py to understand the session registry, then inspect the routes in src/api/agent.py. Run the two-call check described in the issue, reusing one session_id and then trying a new one. Done means prior context is retained for reused IDs, new IDs start clean, and the history endpoint returns chronological steps.

Written by the indexing model from the issue text.