feat: add optional You.com search tool

[mouse-value-add]

Why this is useful

• Comet MCP already helps with live browsing, so a lightweight web search tool fits the workflow when the model just needs external context without opening a browser session.
• I kept it optional and behind YDC_API_KEY so existing behavior stays unchanged.

What changed

• Added a youcom_search MCP tool backed by You.com Search.
• Wired the same tool into the HTTP bridge for parity.
• Added a small shared helper for request/response handling.
• Documented the tool and env var in the README.
• Added focused unit tests for success, missing-key, and upstream error handling.

Setup

• Set YDC_API_KEY to enable the tool.
• No config change is required if you do not want this tool.

Usage

• youcom_search {"query":"playwright locator best practices","count":5}

Validation

• npm install
• npm run build
• npm run test:unit -- tests/unit/youcom.test.ts

Fallback/error handling

• Missing YDC_API_KEY returns a clear setup error.
• Upstream HTTP failures surface the status and message without crashing the server.

Tracking

• [Integration] RapierCraft/Perplexity-Comet-MCP - optional You.com search integration youdotcom-oss/integration-tracking#44