Preserve existing managed Claude model defaults

[lilly-luo]

Summary

• preserve Claude family defaults already present in OS-managed settings
• add defaults for families missing from OS-managed settings
• allow explicit Coding Agent Config family values to overwrite existing defaults
• keep private ucode settings behavior unchanged

Stack

• depends on merge preexisting anthropic custom headers in managed settings + ucode ones #442

Testing

• pytest tests/test_agent_claude.py tests/test_managed_resolve.py -q (196 passed)

• Ruff checks passed for all changed files

• manual testing - no coding agent config
  start with the following in my /etc/claude-code/managed-settings.json (no coding agent config)

    "ANTHROPIC_DEFAULT_OPUS_MODEL": "system.ai.claude-opus-5[1m]",
    "ANTHROPIC_DEFAULT_SONNET_MODEL": "system.ai.claude-sonnet-4-6[1m]",
    "ANTHROPIC_DEFAULT_HAIKU_MODEL": "system.ai.claude-haiku-5",

run uv run ucode claude --refresh (confirmed ucode claude --refresh resets my sonnet to sonnet 5)

confirming the models are preserved (desired behavior, sonnet not reset to sonnet 5):

    "ANTHROPIC_DEFAULT_OPUS_MODEL": "system.ai.claude-opus-5[1m]",
    "ANTHROPIC_DEFAULT_SONNET_MODEL": "system.ai.claude-sonnet-4-6[1m]",
    "ANTHROPIC_DEFAULT_HAIKU_MODEL": "system.ai.claude-haiku-5",

• manual testing - coding agent config
  1/ set sonnet 5 as my coding agent config sonnet version
  2/ updated my /etc/claude-code/managed-settings.json to have sonnet-4-6
  3/ confirmed my ANTHROPIC_DEFAULT_SONNET_MODEL gets overwritten to sonnet 5