[ug] Preserve unmanaged Claude family defaults

[lilly-luo]

Summary

• preserve existing ANTHROPIC_DEFAULT_<FAMILY>_MODEL values when no Coding Agent Config is present
• continue filling family defaults that are absent
• keep managed-config defaults authoritative
• cover private and OS-managed Claude settings with unit regressions
• add a live integration regression against one real workspace verified to have no managed config

Context

PR #444 preserved existing managed-file family defaults, including without a Coding Agent Config. PR #709 later introduced cross-workspace ownership tracking and intentionally re-derived defaults that matched ug's last write. That allowed managed workspace transitions to clear stale defaults, but also caused unmanaged launches to replace preexisting family defaults. This change restores the unmanaged preservation boundary without changing managed-config reconciliation.

The dedicated test_unmanaged_claude_preserves_preexisting_family_defaults live test verifies that its real workspace publishes no CodingAgentConfig, seeds /etc/claude-code/managed-settings.json with all Claude family defaults, runs public ug configure, and requires those exact defaults to remain unchanged. It checks settings reconciliation and makes no inference claim.

Testing

• uv run --frozen ruff check src/ tests/
• uv run --frozen ruff format --check src/ tests/
• uv run --frozen ty check src/
• uv run --frozen pytest tests/test_integration_contract.py tests/test_agent_claude.py::TestWriteToolConfigManagedSettings::test_unmanaged_preserves_existing_family_defaults tests/test_managed_model_lifecycle.py::test_claude_model_lifecycle -q (39 passed)
• focused integration collection with the runner dependencies (3 Claude lifecycle tests collected)
• live unmanaged-workspace regression: CI (this host has machine-wide Claude policy, which the integration runner correctly rejects)