config: unify memory and agent config into single module

Both hippocampus/config.rs and agent/config.rs read from the same
config file (~/.config/poc-agent/config.json5). Having two separate
implementations was a footgun — load_context_groups() was duplicated
three times across the codebase.

Merged into src/config.rs:
- Config (memory settings, global get()/reload())
- AppConfig (agent backend/model settings, figment-based loading)
- SessionConfig (resolved agent session, renamed from agent's Config)
- Single ContextGroup/ContextSource definition used everywhere

Eliminated: duplicate load_context_groups(), duplicate ContextGroup
definition in identity.rs, duplicate config file path constants.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/hippocampus/mod.rs

@@ -15,6 +15,5 @@ pub mod spectral;
 pub mod neuro;
 pub mod counters;
 pub mod migrate;
-pub mod config;
 pub mod transcript;
 pub mod memory_search;