feat: add Context.dev web search command

[aadithyanr]

Summary

• add /search using the Context.dev POST /v1/web/search endpoint
• add ten source-linked results to the chat and label web content as untrusted
• support CONTEXT_DEV_API_KEY and the existing --api-key context_dev= flow
• document the command and cover the HTTP request, chat insertion, and missing-key path

Scope

This is deliberately limited to one read-only endpoint. It adds no MCP layer, tool loop, or dependency, and leaves /web as the command for loading an exact URL.

Validation

• 3 focused search tests passed
• 73 search and command regression tests passed
• isort, Black, flake8, and codespell passed
• generated slash-command reference refreshed

Related to #2754.

[CLAassistant]

All committers have signed the CLA.