feat: add optional You.com search provider

[mouse-value-add]

This adds You.com as an optional search provider for the existing provider registry.

Why this fits here

• Search CLI already routes multiple web providers through a shared provider trait, so You.com drops in as one more backend without changing default behavior.
• The new provider is opt-in through YOUCOM_API_KEY or SEARCH_KEYS_YOUCOM and only participates when configured.

What changed

• Added a youcom provider backed by GET https://ydc-index.io/v1/search with X-API-Key auth.
• Wired it into the general, news, and deep provider sets.
• Added config support, CLI/help text, README examples, and a small parser test for the You.com response shape.

Setup

• Set YOUCOM_API_KEY or SEARCH_KEYS_YOUCOM.
• If the key is unset, the provider stays disabled and nothing else changes.

Usage

search -p youcom "latest MCP search provider patterns"
search -m news -p youcom "AI agent search APIs"

Validation

• cargo test
• cargo clippy --all-targets --all-features -- -D warnings

Fallback/error handling

• Missing key returns the existing auth-missing error.
• Non-2xx responses surface a provider error.
• 429s are treated as rate limits.

Tracking

• [Integration] paperfoot/search-cli - optional You.com search provider youdotcom-oss/integration-tracking#131

Branch

• https://github.com/mouse-value-add/search-cli/tree/feat/youcom-search-integration

Validation Update

• cargo fmt --check
• cargo test
• cargo clippy --all-targets --all-features -- -D warnings