feat(persistence): add session save/resume API

- Add with_session_store(), with_file_session_store(), with_session_id(), with_auto_save() to SessionOptions
- Add session_id(), save() to AgentSession
- Add resume_session() to Agent — restores conversation history
- Auto-save after send() when auto_save=true
- 10 new tests (1158 total, all passing)

Author: ZhiXiao-Lin

README.md

@@ -11,7 +11,7 @@ let result = session.send("Refactor auth to use JWT").await?;
 [![Crates.io](https://img.shields.io/crates/v/a3s-code-core.svg)](https://crates.io/crates/a3s-code-core)
 [![Documentation](https://docs.rs/a3s-code-core/badge.svg)](https://docs.rs/a3s-code-core)
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](./LICENSE)
-[![Tests](https://img.shields.io/badge/tests-1148%20passing-brightgreen.svg)](./core/tests)
+[![Tests](https://img.shields.io/badge/tests-1158%20passing-brightgreen.svg)](./core/tests)
 
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@@ -476,7 +476,7 @@ cargo test          # All tests
 cargo test --lib    # Unit tests only
 ```
 
-**Test Coverage:** 1148 tests, 100% pass rate
+**Test Coverage:** 1158 tests, 100% pass rate
 
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