feat: add optional Linkup web corpus ingestion

[sachasimov]

Summary

• Add an optional LinkupWebLoader that turns known public web pages into KnowledgeBase chunks for RAG adapter training.
• Keep Linkup as an optional extra (linear-adapter-trainer[linkup]) with lazy SDK import and offline fake-client tests.
• Document why clean, sourced, AI-agent-optimized web content helps build trusted retrieval corpora and include a runnable linkup_fetch example config.

Test plan

• uv run pytest
• uv run ruff check .
• uv run black --check .

[github-actions]

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[sachasimov]

I have read the CLA Document and I hereby sign the CLA