Runrail 0.4.0

• Add Grok Build as a coding tool (--backend grok), run through its own Agent
  Client Protocol mode with the Grok sign-in already on the machine. Grok never
  asks before it edits files or runs commands, so a thread starts only with
  full access; any other permission mode is refused with a clear message.
• Keep a value entered for an MCP server's variable in the credential vault
  and hand it to that server when it starts, ahead of the same variable in the
  environment. The control server takes it through a write-only endpoint and
  reports only which names have a value; no method reads a value back.
• Read open GitHub issues through the person's own gh, and run a project's
  worktree setup and archive scripts (.runrail/worktree.json) only when the
  caller sends back the hash of the exact file the person approved.