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/orchestrator.py

@@ -20,7 +20,7 @@ from pydantic import BaseModel
 
 from config import settings
 from context_loader import load_context
-from persona import set_persona, validate as validate_persona
+from persona import set_context, validate as validate_persona
 import orchestrator_engine
 
 logger = logging.getLogger(__name__)
@@ -47,6 +47,7 @@ class OrchestrateRequest(BaseModel):
     include_long: bool = True
     include_mid: bool = True
     include_short: bool = True
+    user: str = "scott"
     persona: str = "inara"
 
 
@@ -77,7 +78,8 @@ class JobStatusResponse(BaseModel):
 async def orchestrate(req: OrchestrateRequest) -> OrchestrateResponse:
     """Submit a task to the orchestrator. Returns a job_id to poll."""
     try:
-        set_persona(validate_persona(req.persona))
+        user, persona = validate_persona(req.user, req.persona)
+        set_context(user, persona)
     except ValueError as e:
         from fastapi import HTTPException
         raise HTTPException(status_code=400, detail=str(e))