Remove bundled clinical doctor expert

[Pandakingxbc]

What changed

• Remove the bundled clinical-doctor expert template, including its prompts, references, and skills.
• Update the expert preview-path integration test to use the existing stock-assistant fixture instead of the removed template.

Why

The primary-care clinical doctor expert is product-specific content and should not be bundled in the main Octop distribution. Keeping it in the default expert catalog exposes an unwanted expert to every installation and couples a generic catalog API test to content that is intended to be maintained separately.

Impact

• New and restarted Octop installations no longer list clinical-doctor in the built-in expert catalog.
• Existing unrelated experts and expert preview behavior remain unchanged.
• The separately maintained clinical learning assistant branches are not modified.

Validation

• uv run pytest tests/unit/agents/test_expert_catalog.py tests/integration/test_experts_api.py -q
• Result: 19 passed
• Restarted the local Octop service successfully with the existing data and channel configuration.