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/agent/mod.rs

@@ -19,7 +19,8 @@ macro_rules! dbglog {
 // - tools/     — tool definitions and dispatch
 // - ui_channel — streaming UI communication
 // - runner     — the interactive agent loop
-// - cli, config, context, dmn, identity, log, observe, parsing, tui
+// - cli, context, dmn, identity, log, observe, parsing, tui
+// Config moved to crate::config (unified with memory config)
 
 pub mod api;
 pub mod types;
@@ -29,7 +30,6 @@ pub mod journal;
 
 pub mod runner;
 pub mod cli;
-pub mod config;
 pub mod context;
 pub mod dmn;
 pub mod identity;