Fix pi pin default model and backup settings.json

[asujithan]

Summary

Fixes Pi config management issue from pr #69 : ucode writessettings.json (pinning defaultProvider/defaultModel) but only ever backed up and rolled back models.json. That left two failure modes:

1. A pre-existing user settings.json was silently overwritten with no restore path.
2. If Pi validation failed or the user ran ucode revert, models.json was rolled back but settings.json was orphaned — pinning a provider/model that no longer existed in the restored models.json, making Pi fail to start on the next run.

Changes

• src/ucode/agents/pi.py
  • Add PI_SETTINGS_BACKUP_PATH.
  • _write_settings now calls backup_existing_file(PI_SETTINGS_PATH, PI_SETTINGS_BACKUP_PATH) before the merge, symmetric to the existing models.json backup in write_tool_config.
• src/ucode/agents/__init__.py
  • validate_all_tools now also calls restore_file(PI_SETTINGS_PATH, PI_SETTINGS_BACKUP_PATH, managed) when Pi validation fails, so both managed files roll back together.
• src/ucode/cli.py
  • revert() now restores settings.json alongside models.json and prints a "Pi settings: restored | unchanged" row.

Tests Added

• tests/test_agent_pi.py::TestWriteToolConfig::test_pre_existing_settings_are_backed_up_before_first_write
  — pre-existing settings.json is copied to the backup path before ucode's pin is applied; the merged file retains user keys.
• tests/test_agent_pi.py::TestValidateAllToolsPiRollback::test_failed_pi_validation_rolls_back_settings — drives validate_all_tools with validate_tool stubbed to fail and asserts settings.json is removed on the managed-rollback branch.

Test plan

• uv run ruff check .
• uv run pytest --ignore=tests/test_e2e.py --ignore=tests/test_e2e_user_agent.py — 593 passed
• CI checks