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/bin/poc-agent.rs

@@ -35,8 +35,8 @@ use poc_memory::dbglog;
 use poc_memory::agent::*;
 use poc_memory::agent::runner::{Agent, TurnResult};
 use poc_memory::agent::api::ApiClient;
-use poc_memory::agent::config::{AppConfig, Config};
 use poc_memory::agent::tui::HotkeyAction;
+use poc_memory::config::{self, AppConfig, SessionConfig};
 use poc_memory::agent::ui_channel::{ContextInfo, StatusInfo, StreamTarget, UiMessage};
 
 /// Hard compaction threshold — context is rebuilt immediately.
@@ -120,7 +120,7 @@ enum Command {
 /// and slash commands.
 struct Session {
     agent: Arc<Mutex<Agent>>,
-    config: Config,
+    config: SessionConfig,
     process_tracker: tools::ProcessTracker,
     ui_tx: ui_channel::UiSender,
     turn_tx: mpsc::Sender<(Result<TurnResult>, StreamTarget)>,
@@ -149,7 +149,7 @@ struct Session {
 impl Session {
     fn new(
         agent: Arc<Mutex<Agent>>,
-        config: Config,
+        config: SessionConfig,
         process_tracker: tools::ProcessTracker,
         ui_tx: ui_channel::UiSender,
         turn_tx: mpsc::Sender<(Result<TurnResult>, StreamTarget)>,
@@ -817,25 +817,9 @@ impl Session {
         self.send_context_info();
     }
 
-    /// Load context_groups from the shared config file.
-    fn load_context_groups(&self) -> Vec<identity::ContextGroup> {
-        let config_path = dirs::home_dir()
-            .unwrap_or_else(|| PathBuf::from("."))
-            .join(".config/poc-agent/config.json5");
-        
-        if let Ok(content) = std::fs::read_to_string(&config_path) {
-            let config: Result<serde_json::Value, _> = json5::from_str(&content);
-            if let Ok(config) = config {
-                if let Some(memory) = config.get("memory") {
-                    if let Some(groups) = memory.get("context_groups") {
-                        if let Ok(context_groups) = serde_json::from_value(groups.clone()) {
-                            return context_groups;
-                        }
-                    }
-                }
-            }
-        }
-        Vec::new()
+    /// Get context_groups from the unified config.
+    fn load_context_groups(&self) -> Vec<config::ContextGroup> {
+        config::get().context_groups.clone()
     }
 
     /// Send context loading info to the TUI debug screen.
@@ -885,7 +869,7 @@ impl Session {
 // --- Event loop ---
 
 async fn run(cli: cli::CliArgs) -> Result<()> {
-    let (config, _figment) = config::load(&cli)?;
+    let (config, _figment) = config::load_session(&cli)?;
 
     // Wire config.debug to the POC_DEBUG env var so all debug checks
     // throughout the codebase (API, SSE reader, diagnostics) see it.