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/cron_runner.py

@@ -45,12 +45,12 @@ _DOW = {
 # Storage
 # ---------------------------------------------------------------------------
 
-def crons_path(persona: str | None = None) -> Path:
-    return _persona_path(persona) / "CRONS.json"
+def crons_path(username: str | None = None, persona: str | None = None) -> Path:
+    return _persona_path(username, persona) / "CRONS.json"
 
 
-def load_crons(persona: str | None = None) -> list[dict]:
-    p = crons_path(persona)
+def load_crons(username: str | None = None, persona: str | None = None) -> list[dict]:
+    p = crons_path(username, persona)
     if not p.exists():
         return []
     try:
@@ -60,9 +60,11 @@ def load_crons(persona: str | None = None) -> list[dict]:
         return []
 
 
-def save_crons(crons: list[dict], persona: str | None = None) -> None:
+def save_crons(crons: list[dict],
+               username: str | None = None,
+               persona: str | None = None) -> None:
     import json
-    crons_path(persona).write_text(json.dumps(crons, indent=2) + "\n")
+    crons_path(username, persona).write_text(json.dumps(crons, indent=2) + "\n")
 
 
 # ---------------------------------------------------------------------------
@@ -134,7 +136,7 @@ async def run_job(job: dict) -> None:
     label = job.get("label", job.get("id", "cron"))
     section = f"\n## {label} — {_now_label()}\n\n{payload}\n"
 
-    p_root = _persona_path(job.get("persona"))
+    p_root = _persona_path(job.get("user"), job.get("persona"))
 
     if job_type == "remind":
         p = p_root / "REMINDERS.md"
@@ -153,10 +155,10 @@ async def run_job(job: dict) -> None:
         return
 
     # Record last_run in the right persona's CRONS.json
-    persona = job.get("persona")
-    crons = load_crons(persona)
+    u, p = job.get("user"), job.get("persona")
+    crons = load_crons(u, p)
     for c in crons:
         if c["id"] == job["id"]:
             c["last_run"] = datetime.now(timezone.utc).isoformat()
             break
-    save_crons(crons, persona)
+    save_crons(crons, u, p)