feat(ecc2): implement live output streaming per agent (#774)

- PTY output capture via tokio::process with stdout/stderr piping
- Ring buffer (1000 lines) per session
- Output pane wired to show selected session with auto-scroll
- Broadcast channel for output events

ecc2/src/session/mod.rs

@@ -1,5 +1,7 @@
 pub mod daemon;
 pub mod manager;
+pub mod output;
+pub mod runtime;
 pub mod store;
 
 use chrono::{DateTime, Utc};