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/config.py

@@ -27,8 +27,7 @@ class Settings(BaseSettings):
     agent_name: str = "Inara"
     user_name: str = "Scott"
 
-    personas_dir: Path = Path("../personas")
-    inara_dir: Path = Path("../personas/inara")  # legacy — use personas_dir
+    home_dir: Path = Path("../home")
     sessions_dir: Path = Path("./data/sessions")
     default_model: str = "claude-sonnet-4-6"
     default_tier: int = 2
@@ -74,15 +73,11 @@ class Settings(BaseSettings):
 
     model_config = SettingsConfigDict(env_file=".env", env_file_encoding="utf-8", extra="ignore")
 
-    def personas_root(self) -> Path:
-        """Resolve personas_dir relative to this file's location if not absolute."""
-        if self.personas_dir.is_absolute():
-            return self.personas_dir
-        return (Path(__file__).parent / self.personas_dir).resolve()
-
-    def inara_path(self) -> Path:
-        """Legacy helper — returns the inara persona directory. Prefer persona_path()."""
-        return self.personas_root() / "inara"
+    def home_root(self) -> Path:
+        """Resolve home_dir relative to this file's location if not absolute."""
+        if self.home_dir.is_absolute():
+            return self.home_dir
+        return (Path(__file__).parent / self.home_dir).resolve()
 
     def sessions_path(self) -> Path:
         """Resolve sessions_dir relative to this file's location if not absolute."""