poc-agent: read context_groups from config instead of hardcoded list

- Remove MEMORY_FILES constant from identity.rs
- Add ContextGroup struct for deserializing from config
- Load context_groups from ~/.config/poc-agent/config.json5
- Check ~/.config/poc-agent/ first for identity files, then project/global
- Debug screen now shows what's actually configured

This eliminates the hardcoded duplication and makes the debug output
match what's in the config file.

poc-memory/src/agents/api.rs

@@ -179,7 +179,11 @@ pub fn call_api_with_tools_sync(
                 .enable_all()
                 .build()
                 .map_err(|e| format!("tokio runtime: {}", e))?;
-            rt.block_on(call_api_with_tools(agent, prompt, log))
+            let prov = format!("agent:{}", agent);
+            rt.block_on(
+                crate::store::TASK_PROVENANCE.scope(prov,
+                    call_api_with_tools(agent, prompt, log))
+            )
         }).join().unwrap()
     })
 }