feat: session naming, username/persona rename, help page, contrast fixes

- Session name field: PATCH /sessions/{id} endpoint, inline rename button in UI
- Persona rename: inline ✏ toggle form in settings, POST /settings/persona/rename
- Username rename: inline form in settings, POST /settings/username (renames home dir, forces re-login)
- Help page: dedicated /help route replacing modal, collapsible sections
- Per-persona isolation: files.py and session_store.py now scope to correct user/persona
- Contrast/visibility: muted text bumped to slate-400+, session rename btn at 0.4 opacity

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

cortex/routers/chat.py

@@ -6,7 +6,7 @@ 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, generate_session_id, delete as delete_session
+from session_store import load as load_session, save as save_session, list_all, generate_session_id, delete as delete_session, rename as rename_session
 from config import settings
 from persona import set_context, validate as validate_persona
 import event_bus
@@ -171,6 +171,24 @@ async def list_sessions(
     return {"sessions": list_all()}
 
 
+class SessionRename(BaseModel):
+    name: str
+
+
+@router.patch("/sessions/{session_id}")
+async def rename_session_endpoint(
+    session_id: str,
+    req: SessionRename,
+    user: str = Query("scott"),
+    persona: str = Query("inara"),
+) -> dict:
+    _set_ctx(user, persona)
+    found = rename_session(session_id, req.name.strip())
+    if not found:
+        raise HTTPException(status_code=404, detail=f"Session {session_id} not found")
+    return {"ok": True, "session_id": session_id, "name": req.name.strip()}
+
+
 @router.delete("/sessions/{session_id}")
 async def delete_session_endpoint(
     session_id: str,