Remove app/ai folder and inline extracted dependencies

[Copilot]

Removes the legacy app/ai pipeline and all associated code, replacing the single cross-module dependency (extract_links) with an inline implementation.

Changes

• Deleted app/ai/: Entire legacy pipeline folder including pipeline steps, adjudication, claim extraction, retrieval, threads, utils, and all associated tests/docs
• Deleted app/api/endpoints/research.py and test.py: Endpoints that depended on the old pipeline
• app/agentic_ai/nodes/format_input.py: Replaced import of extract_links from deleted module with a local regex-based implementation
• app/api/endpoints/__init__.py / app/main.py: Removed registrations for the now-deleted research and test routers
• Added app/agentic_ai/tests/nodes/test_extract_links.py: Tests for the new inline extract_links
• Deleted app/agentic_ai/context/factcheckapi/tests/google_factcheck_gatherer_test.py: Test for removed gatherer

# extract_links moved from app/ai/pipeline/link_context_expander into format_input.py
def extract_links(text: str) -> list[str]:
    url_pattern = r'https?://[^\s<>"{}|\\^`\[\]]+'
    urls = re.findall(url_pattern, text)
    # strips trailing punctuation and deduplicates
    ...

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