refactor: remove AIEOS identity support

- Remove src/identity/ directory (aieos.rs, mod.rs)
- Remove IdentityConfig struct and identity field from Config
- Remove build_system_prompt_with_identity and load_aieos_from_config functions
- Remove AIEOS-related imports from channels/mod.rs
- Remove identity module declarations from main.rs and lib.rs
- Remove AIEOS tests from config/schema.rs
- Keep OpenClaw markdown-based identity as the only supported format

This simplifies the codebase by removing unused AIEOS complexity.
All 832 tests pass.

src/lib.rs

@@ -13,7 +13,6 @@
 
 pub mod config;
 pub mod heartbeat;
-pub mod identity;
 pub mod memory;
 pub mod observability;
 pub mod providers;