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/memory_distiller.py

@@ -7,6 +7,8 @@ Inara tiered memory distillation.
 """
 import logging
 from datetime import datetime
+from pathlib import Path
+from config import settings
 from persona import persona_path as _persona_path
 
 logger = logging.getLogger(__name__)
@@ -23,14 +25,14 @@ def _read(path: Path) -> str:
     return path.read_text() if path.exists() else ""
 
 
-def distill_short(persona: str | None = None) -> dict:
+def distill_short(username: str | None = None, persona: str | None = None) -> dict:
     """
     Roll the most recent session log files into MEMORY_SHORT.md.
     No LLM involved — pure aggregation with budget truncation.
     Files are included newest-first until the budget is reached,
     then written in chronological order (oldest first).
     """
-    inara_dir = _persona_path(persona)
+    inara_dir = _persona_path(username, persona)
     sessions_dir = inara_dir / "sessions"
     budget = _budget_chars(settings.memory_budget_short)
 
@@ -72,13 +74,13 @@ def distill_short(persona: str | None = None) -> dict:
     }
 
 
-async def distill_mid(persona: str | None = None) -> dict:
+async def distill_mid(username: str | None = None, persona: str | None = None) -> dict:
     """
     Ask the LLM to summarize MEMORY_SHORT.md → MEMORY_MID.md.
     """
     from llm_client import complete
 
-    inara_dir = _persona_path(persona)
+    inara_dir = _persona_path(username, persona)
     short_content = _read(inara_dir / "MEMORY_SHORT.md")
 
     if not short_content.strip() or "Not yet populated" in short_content:
@@ -116,13 +118,13 @@ async def distill_mid(persona: str | None = None) -> dict:
     }
 
 
-async def distill_long(persona: str | None = None) -> dict:
+async def distill_long(username: str | None = None, persona: str | None = None) -> dict:
     """
     Ask the LLM to integrate MEMORY_MID.md into MEMORY_LONG.md.
     """
     from llm_client import complete
 
-    inara_dir = _persona_path(persona)
+    inara_dir = _persona_path(username, persona)
     long_content = _read(inara_dir / "MEMORY_LONG.md")
     mid_content = _read(inara_dir / "MEMORY_MID.md")