Move orchestration from persona to profiles; tune per-profile delegation strategy ...

persona.txt now contains only: identity, profile switching, workspace,
response hygiene, memory, and documentation. All orchestration instructions
removed from the global scope.

Each profile gets its own orchestration model:
- secretary: full orchestrator — delegate any 2+ tool-call sub-task to agents,
  scratchpad as blackboard, todo for milestone tracking
- server_admin: heavy orchestrator — one agent per host / per concern,
  parallel delegation, diagnose-before-act discipline
- developer: builder + research delegation — implementation always inline,
  spawn only for large API/codebase research tasks

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Restructure profiles: directory-based format with config.json + system_prompt.txt ...

Each profile is now a subdirectory under navi/profiles/ containing:
  config.json       — model, temperature, enabled_tools, and other settings
  system_prompt.txt — raw system prompt, editable without touching Python

Added navi/profiles/loader.py for auto-discovery of profile directories.
Removed individual profile .py files (secretary, server_admin, smart_home, developer).
profiles/__init__.py now simply calls load_profiles_from_dir() at import time.

New profiles can be added by creating a directory with the two required files —
no Python changes needed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>