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>

home/holly/persona/tina/CRONS.json

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+[]

home/holly/persona/tina/IDENTITY.md

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+# [Agent Name TBD] — Identity
+
+**Name:** [Choose a name]
+**Role:** Personal AI assistant
+**User:** Holly
+
+*Choose a name and define this agent's identity, backstory, and how she
+introduces herself. Then update AGENT_NAME in cortex/.env.holly to match.*

home/holly/persona/tina/MEMORY_LONG.md

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+# MEMORY_LONG.md — [Agent Name TBD] Long-Term Memory
+
+*Not yet populated — will be auto-generated after distillation runs.*

home/holly/persona/tina/MEMORY_MID.md

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+# MEMORY_MID.md — [Agent Name TBD] Mid-Term Memory
+
+*Not yet populated.*

home/holly/persona/tina/MEMORY_SHORT.md

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+# MEMORY_SHORT.md — [Agent Name TBD] Recent Session Digest
+
+*Not yet populated.*

home/holly/persona/tina/PROTOCOLS.md

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+# [Agent Name TBD] — Protocols
+
+*Define Holly's behavioural rules, response style, and any constraints here.*
+
+---
+
+**Placeholder** — fill this in before starting Holly's instance.

home/holly/persona/tina/SOUL.md

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+# [Agent Name TBD] — Soul & Values
+
+*Define Holly's personality, values, communication style, and what makes her
+distinct from other AI assistants here.*
+
+---
+
+**Placeholder** — fill this in before starting Holly's instance.

home/holly/persona/tina/TASKS.json

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+[]

home/holly/persona/tina/USER.md

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+# User Profile — Holly
+
+*Document Holly's preferences, interests, and context here so the agent
+can personalise responses over time.*
+
+---
+
+**Placeholder** — fill this in before starting Holly's instance.