examples: add local ART-E email search task

[Vinzz2303]

Summary

• Adds a lightweight local ART-E email search example under examples/art_e
• Includes deterministic email scenarios, search/read helpers, answer scoring, rollout loop, and a training entrypoint
• Adds unit tests for the local scenario helpers and links the example from the root README

Motivation

The README points users to the ART-E email-search agent, but the main repo did not include a small local example that can be inspected without external email infrastructure. This adds a portable starter task for experimenting with ART-style email search rollouts before scaling to the full ART-E setup.

What changed

• examples/art_e/scenarios.py: local inbox fixtures, search/read helpers, JSON command parsing, and reward scoring
• examples/art_e/rollout.py: multi-turn ART trajectory using <search>, <read>, and <answer> commands
• examples/art_e/train.py: LocalBackend training loop for the ART-E style task
• examples/art_e/README.md: quickstart and file overview
• tests/test_art_e_example.py: helper-level tests for search, read, scoring, and command parsing
• README.md: adds the local ART-E example to the notebooks/examples table

Testing

Not run locally because this Windows environment does not currently have python or uv available in PATH. The added tests are intentionally small and should run with the repo test suite once dependencies are installed.