Fix NAVI_PERSONA not loading: move persona to persona.txt ...

python-dotenv cannot parse multi-line env values that contain blank lines or
lines resembling key=value assignments (like the write_tool code examples).
The persona was silently dropped, leaving Navi with no identity.

Solution: navi_persona_file setting (default empty) — if set, the persona is
read from the specified file at startup via a model_validator. The .env now
points to persona.txt instead of embedding the multi-line string.

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