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/commands.js

@@ -5,6 +5,7 @@
 const COMMANDS=[
   {name:'help',      desc:'List available commands',             fn:cmdHelp},
   {name:'clear',     desc:'Clear conversation messages',         fn:cmdClear},
+  {name:'compact',   desc:'Compress conversation context',       fn:cmdCompact},
   {name:'model',     desc:'Switch model (e.g. /model gpt-4o)',  fn:cmdModel,     arg:'model_name'},
   {name:'workspace', desc:'Switch workspace by name',            fn:cmdWorkspace, arg:'name'},
   {name:'new',       desc:'Start a new chat session',            fn:cmdNew},
@@ -99,6 +100,15 @@ async function cmdNew(){
   showToast('New session created');
 }
 
+function cmdCompact(){
+  // Send as a regular message to the agent -- the agent's run_conversation
+  // preflight will detect the high token count and trigger _compress_context.
+  // We send a user message so it appears in the conversation.
+  $('msg').value='Please compress and summarize the conversation context to free up space.';
+  send();
+  showToast('Requesting context compression...');
+}
+
 async function cmdUsage(){
   const next=!window._showTokenUsage;
   window._showTokenUsage=next;