feat: multi-user/multi-persona support with two-level home directory layout

Restructures persona storage from a flat personas/{name}/ layout to
home/{username}/persona/{name}/, mirroring Linux home directories.

Changes:
- persona.py: two ContextVars (user + persona), Linux-style name validation,
  set_context(), get_user(), get_persona(), validate(), list_users(),
  list_user_personas(); persona_path() takes (username, name)
- config.py: replaces personas_dir with home_dir + home_root()
- git mv personas/inara → home/scott/persona/inara (history preserved)
- home/holly/persona/tina/: Holly's persona stub added
- cron_runner.py: all storage functions take (username, persona) params
- tools/cron.py: stamps user + persona on jobs; APScheduler IDs are
  {user}:{persona}:{job_id} to prevent collisions across users
- memory_distiller.py: distill_short/mid/long take (username, persona);
  added missing Path + settings imports
- scheduler.py: _load_user_crons() iterates home/*/persona/* (two-level)
- routers/chat.py, orchestrator.py: user field added; set_context() called
- tests/conftest.py: home_root fixture with two-level structure;
  patches home_dir instead of personas_dir
- tests/test_persona.py: fully rewritten for two-level API
- tests/test_api_files.py: updated fixture name and path
- .env.default: documents HOME_DIR setting; scrubs stale API key
- CLAUDE.md, README.md: directory maps updated for new layout

All 80 tests pass.

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

cortex/routers/chat.py

@@ -8,7 +8,7 @@ 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 config import settings
-from persona import set_persona, validate as validate_persona
+from persona import set_context, validate as validate_persona
 import event_bus
 
 
@@ -23,6 +23,7 @@ class ChatRequest(BaseModel):
     include_long: bool = True
     include_mid: bool = True
     include_short: bool = True
+    user: str = "scott"
     persona: str = "inara"
 
 
@@ -52,7 +53,8 @@ async def _stream_chat(req: ChatRequest):
       data: {"type": "error", "message": "..."}
     """
     try:
-        set_persona(validate_persona(req.persona))
+        user, persona = validate_persona(req.user, req.persona)
+        set_context(user, persona)
     except ValueError as e:
         yield f"data: {json.dumps({'type': 'error', 'message': str(e)})}\n\n"
         return