Add edit/delete history, named sessions, scroll fix, systemd service

- Edit/delete individual messages from session context with inline editing
  (Ctrl+Enter saves, Escape cancels); changes sync to backend via PUT /history
- PUT /history/{session_id} endpoint to replace full message list
- Named sessions: readable slugs (e.g. quiet-spring) instead of UUID fragments
- Scroll no longer snaps to bottom when user has scrolled up to read history
- cortex.service: systemd unit for auto-start and restart-on-failure

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/routers/chat.py

@@ -1,13 +1,12 @@
 import asyncio
 import json
-import uuid
 from fastapi import APIRouter, HTTPException
 from fastapi.responses import StreamingResponse
 from pydantic import BaseModel
 from context_loader import load_context
 from llm_client import complete
 from session_logger import log_turn
-from session_store import load as load_session, save as save_session, list_all
+from session_store import load as load_session, save as save_session, list_all, generate_session_id
 from config import settings
 
 
@@ -30,6 +29,10 @@ class NoteRequest(BaseModel):
     note: str
 
 
+class HistoryUpdate(BaseModel):
+    messages: list[dict]
+
+
 async def _stream_chat(req: ChatRequest):
     """
     SSE generator: sends keepalive events every 3s while the LLM works,
@@ -42,7 +45,7 @@ async def _stream_chat(req: ChatRequest):
               "backend": "...", "fallback_used": bool}
       data: {"type": "error", "message": "..."}
     """
-    session_id = req.session_id or str(uuid.uuid4())[:8]
+    session_id = req.session_id or generate_session_id()
     tier = req.tier or settings.default_tier
 
     system_prompt = load_context(tier)
@@ -131,6 +134,13 @@ async def list_sessions() -> dict:
     return {"sessions": list_all()}
 
 
+@router.put("/history/{session_id}")
+async def replace_history(session_id: str, req: HistoryUpdate) -> dict:
+    """Replace the full message list for a session (used by edit/delete UI)."""
+    save_session(session_id, req.messages)
+    return {"ok": True, "session_id": session_id}
+
+
 @router.post("/note")
 async def add_note(req: NoteRequest) -> dict:
     """Inject a public note into session history so the LLM sees it next turn."""