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/lib.rs

@@ -13,6 +13,9 @@ pub mod hippocampus;
 // Autonomous agents
 pub mod subconscious;
 
+// Unified configuration
+pub mod config;
+
 // Shared utilities
 pub mod util;
 
@@ -31,7 +34,7 @@ pub mod memory_capnp {
 pub use hippocampus::{
     store, graph, lookups, cursor, query,
     similarity, spectral, neuro, counters,
-    config, transcript, memory_search, migrate,
+    transcript, memory_search, migrate,
 };
 pub use hippocampus::query::engine as search;
 pub use hippocampus::query::parser as query_parser;