Preserve existing managed Claude model defaults

[lilly-luo]

Summary

Currently, we override managed settings with ucode model family defaults in Claude Code. Instead, the hierarchy should be:
1/ coding agent config model family default (highest pri)
2/ existing model family default in the managed setting file
3/ ucode default (lowest pri)

this PR forces that hierarchy. for fable: if fable is enabled, we follow the same hierarchy above. if fable is not enabled, drop fable from the managed config settings.

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

• manual testing of fable
  fable NOT enabled, coding agent config has fable -> should not have fable in the managed settings
  fable NOT enabled, existing fable model in managed settings -> should not have fable in the managed settings
  fable NOT enabled, no fable model in managed settings -> should not have fable in the managed settings
  fable enabled, coding agent config has fable -> should have coding agent config's fable version
  fable enabled, existing fable model in managed settings -> should have preexisting fable version
  fable enabled, no fable model in managed settings -> should have ucode's default fable version