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/identity.rs

@@ -5,17 +5,9 @@
 // from the shared config file.
 
 use anyhow::Result;
-use serde::Deserialize;
 use std::path::{Path, PathBuf};
 
-#[derive(Debug, Clone, Deserialize)]
-pub struct ContextGroup {
-    pub label: String,
-    #[serde(default)]
-    pub keys: Vec<String>,
-    #[serde(default)]
-    pub source: Option<String>, // "file" or "journal"
-}
+use crate::config::{ContextGroup, ContextSource};
 
 /// Read a file if it exists and is non-empty.
 fn read_nonempty(path: &Path) -> Option<String> {
@@ -96,12 +88,12 @@ fn load_memory_files(cwd: &Path, memory_project: Option<&Path>, context_groups:
 
     // Load from context_groups
     for group in context_groups {
-        match group.source.as_deref() {
-            Some("journal") => {
+        match group.source {
+            ContextSource::Journal => {
                 // Journal loading handled separately
                 continue;
             }
-            Some("file") | None => {
+            ContextSource::File | ContextSource::Store => {
                 // File source - load each key as a file
                 for key in &group.keys {
                     let filename = format!("{}.md", key);
@@ -113,9 +105,7 @@ fn load_memory_files(cwd: &Path, memory_project: Option<&Path>, context_groups:
                     }
                 }
             }
-            Some(other) => {
-                eprintln!("Unknown context group source: {}", other);
-            }
+            // All variants covered
         }
     }