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>

api/streaming.py

@@ -12,6 +12,7 @@ from pathlib import Path
 
 from api.config import (
     STREAMS, STREAMS_LOCK, CANCEL_FLAGS, CLI_TOOLSETS,
+    LOCK, SESSIONS, SESSION_DIR,
     _get_session_agent_lock, _set_thread_env, _clear_thread_env,
     resolve_model_provider,
 )
@@ -206,6 +207,40 @@ def _run_agent_streaming(session_id, msg_text, model, workspace, stream_id, atta
                 persist_user_message=msg_text,
             )
             s.messages = result.get('messages') or s.messages
+
+            # ── Handle context compression side effects ──
+            # If compression fired inside run_conversation, the agent may have
+            # rotated its session_id. Detect and fix the mismatch so the WebUI
+            # continues writing to the correct session file.
+            _agent_sid = getattr(agent, 'session_id', None)
+            _compressed = False
+            if _agent_sid and _agent_sid != session_id:
+                old_sid = session_id
+                new_sid = _agent_sid
+                # Rename the session file
+                old_path = SESSION_DIR / f'{old_sid}.json'
+                new_path = SESSION_DIR / f'{new_sid}.json'
+                s.session_id = new_sid
+                with LOCK:
+                    if old_sid in SESSIONS:
+                        SESSIONS[new_sid] = SESSIONS.pop(old_sid)
+                if old_path.exists() and not new_path.exists():
+                    try:
+                        old_path.rename(new_path)
+                    except OSError:
+                        pass
+                _compressed = True
+            # Also detect compression via the result dict or compressor state
+            if not _compressed:
+                _compressor = getattr(agent, 'context_compressor', None)
+                if _compressor and getattr(_compressor, 'compression_count', 0) > 0:
+                    _compressed = True
+            # Notify the frontend that compression happened
+            if _compressed:
+                put('compressed', {
+                    'message': 'Context auto-compressed to continue the conversation',
+                })
+
             # Stamp 'timestamp' on any messages that don't have one yet
             _now = time.time()
             for _m in s.messages: