Add MCP remote CI tools

[uchebnick]

Summary

• Add MCP tools for remote CI workflows: create_ci_workflow, bind_remote_ci, remote_sync_index, and remote_download_index.
• Wire the tools through typed MCP params/results, text renderers, and the CLI MCP backend.
• Update the installed Codex skill, README, npm README, Mintlify MCP docs, and npm MCP smoke expectations.

Tests

• go test ./internal/mcp ./internal/cli
• go test ./...
• npm test --prefix npm/unch
• UNCH_MCP_COMMAND=<fresh local wrapper> node npm/unch/scripts/mcp-smoke.js
• real stdio smoke via go run ./cmd/unch start mcp for tools/list, create_ci_workflow, and bind_remote_ci