feat: handle auto-compaction side effects + /compact command

The agent's run_conversation() already triggers context compression
internally, but the WebUI was unaware of the side effects:

1. Session ID rotation: compression creates a new session_id inside
   the agent. The WebUI kept writing to the old session file, causing
   silent data loss. Fix: detect agent.session_id mismatch after
   run_conversation(), rename the session file, and update in-memory
   caches.

2. No user notification: compression was invisible. Fix: emit a
   'compressed' SSE event when compression is detected. Frontend shows
   a system message and toast.

3. No manual control: Fix: add /compact slash command that sends a
   message to the agent requesting context compression. Shows in the
   autocomplete dropdown.

Detection works two ways:
- agent.session_id != original session_id (ID rotation)
- agent.context_compressor.compression_count > 0 (compressor state)

Closes #90

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/messages.js

@@ -177,6 +177,17 @@ async function send(){
       renderSessionList();setBusy(false);setStatus('');
     });
 
+    source.addEventListener('compressed',e=>{
+      // Context was auto-compressed during this turn -- show a system message
+      if(!S.session||S.session.session_id!==activeSid) return;
+      try{
+        const d=JSON.parse(e.data);
+        const sysMsg={role:'assistant',content:'*[Context was auto-compressed to continue the conversation]*'};
+        S.messages.push(sysMsg);
+        showToast(d.message||'Context compressed');
+      }catch(err){}
+    });
+
     source.addEventListener('apperror',e=>{
       // Application-level error sent explicitly by the server (rate limit, crash, etc.)
       // This is distinct from the SSE network 'error' event below.