feat: add optional you.com search provider

[mouse-value-add]

This adds You.com as an optional search provider in the existing provider fan-out. I kept it aligned with the current provider pattern so default behavior stays the same unless the key is configured.

What changed:

• Added a youcom provider that calls POST https://ydc-index.io/v1/search with X-API-Key auth.
• Mapped You.com web/news results into the existing SearchResult shape.
• Wired the provider into the provider registry, config check / doctor, agent-info, and the CLI help text.
• Added focused unit tests for the new provider and updated the integration tests that track provider lists.

Setup:

• Set YDC_API_KEY.
• SEARCH_KEYS_YOUCOM is also accepted through the existing config overlay.
• search config set keys.youcom <key> stores the key locally.

Usage:

• search "rust async runtimes"
• search -p youcom -q "rust async runtimes"
• search -m news -q "AI agents"

Fallback/error handling:

• If no key is configured, You.com stays disabled and the rest of the search fan-out is unchanged.
• API or parsing failures are reported as normal provider failures.

Validation:

• cargo fmt --all
• cargo test
• cargo test --test integration -- --nocapture

Tracking issue: youdotcom-oss/integration-tracking#223