merge preexisting anthropic custom headers in managed settings + ucode ones

[lilly-luo]

Summary

When ucode writes its Anthropic custom headers to the managed config, it should preserve all the existing custom headers that were there previously EXCEPT the headers that need to be explicitly added by ucode.

currently, ucode blindly overwrites all anthropic custom headers with its own values.

In this PR, we:
1/ load up the custom headers previously present in the managed config
2/ put in the ucode headers. ucode header values take priority over what's existing in the managed config.
3/ the non-overlapping custom headers from the managed config remain present
4/ save those custom headers to managed settings.

Testing

• uv run ruff check src/ucode/agents/claude.py tests/test_agent_claude.py

• uv run pytest tests/test_agent_claude.py -q (127 passed)

• manual testing
  started /etc/claude-code/managed-settings.json with:

"ANTHROPIC_CUSTOM_HEADERS": "x-databricks-use-coding-agent-mode: true\nUser-Agent: ucode/FAKEVERSION claude/2.1.258\nmeep: lala",

ran uv run ucode claude

after /etc/claude-code/managed-settings.json has:

"ANTHROPIC_CUSTOM_HEADERS": "x-databricks-use-coding-agent-mode: true\nUser-Agent: ucode/0.1.0+41.gd09c080 claude/2.1.258\nmeep: lala",