docs: add hosted PipeOps MCP setup guide

[9trocode]

Summary

• add an end-to-end hosted MCP guide for Codex and other Streamable HTTP clients
• document least-privilege service-token setup, workspace isolation, and blocked sensitive operations
• add MCP discovery to the docs overview and CLI references
• remove the stale proxy utility entry from the command overview

Verification

• yarn build
• confirmed canonical route: /docs/integrations/pipeops-mcp
• live endpoint smoke check: health and protected-resource metadata return successfully; unauthenticated MCP initialization returns 401

Existing warning

Docusaurus still reports the pre-existing broken anchor from /docs/projects/gitops to /docs/projects/project-settings#environment-variable; the build succeeds.