Import Pi provider configuration

[alejandro-ao]

Summary

Adds a best-effort Pi configuration importer so users can migrate provider/model setup into Tau without starting from scratch.

Closes #293.

What changed

• Added tau config import-pi <path> for JSON/TOML Pi config files.
• Added --dry-run, --yes, and --default-pi-config import flags.
• Added conservative provider/model mapping with warnings for raw secrets and unmapped fields.
• Protected existing ~/.tau/providers.json unless --yes is provided.
• Added unit/CLI coverage for happy path, invalid/missing config, dry run, and overwrite protection.
• Added migration docs and architecture notes.

Tests

uv run ruff format .
uv run ruff check .
uv run mypy
uv run pytest

All passed locally.