feat: add opt-in Parallel Search MCP

[georgeatparallel]

OpenTag already supports optional MCP sources alongside its Tavily web-search tool. This adds a no-account, no-key option for current web search and URL fetching without changing any existing source or default.

What changed

• Add an off-by-default Parallel Search MCP source enabled by PARALLEL_MCP_URL=https://search.parallel.ai/mcp.
• Prefix its tools as parallel_web_search and parallel_web_fetch, so Tavily's existing web_search remains available when both are configured.
• Advertise Parallel web capabilities only after both tools are successfully discovered; source failures keep the existing no-web behavior.
• Carry the non-secret opt-in through local setup, Railway, and AWS configuration.

Validation

• cd agent && uv run --frozen pytest (78 passed)
• pnpm check-types
• pnpm test (178 passed)
• node node_modules/railway/dist/iac/bin.js
• pnpm --dir deployment/aws build
• pnpm --dir deployment/aws test (11 passed)
• git diff --check

The tests use fake MCP clients and do not contact the live endpoint.

Disclosure: I work at Parallel.ai, which operates this MCP endpoint.

[mxmzb]

Thanks @georgeatparallel for this. #81 made Parallel the default web search provider in OpenTag, so this opt-in path is no longer needed. Closing as superseded, and thank you for the groundwork.

[georgeatparallel]

Thanks @mxmzb, makes sense. Great to see Parallel as the default in #81. Appreciate you closing the loop.