chore: update AGENT.md file

[maralbahari]

Summary

Update AGENTS.md with concise, repository-specific Rust guidance:

• Document the workspace and agentic-server-core module boundaries.
• Add ownership, error-handling, async, type-safety, and performance practices.
• Encourage borrowing and avoiding unnecessary .clone() calls.
• Require shared JSON helpers in production src/ code while allowing direct serde_json use in tests.
• Document the cassette recorder workflow for replay tests.

Test Plan

• Documentation-only change; no runtime tests required.