Compiler does not emit GITHUB_HOST in MCP server env for GHES targets #20811
Description
Test result (v0.58.0): Fresh compilation of any workflow with env: GH_HOST in frontmatter produces a lock file with "GITHUB_PERSONAL_ACCESS_TOKEN" in the MCP server JSON env but no "GITHUB_HOST" entry. The MCP server env contains 0 occurrences of GITHUB_HOST. Every lock file requires post-compilation injection.
Problem: The compiled lock file's MCP server JSON config does not include a GITHUB_HOST environment variable. The GitHub MCP Server defaults to api.github.com, so all API calls fail on GHES with 401/404. Users must manually inject "GITHUB_HOST":"https://<ghes-host>" into the MCP server env block after every compilation.
Expected behavior: When env.GH_HOST is set in the workflow .md frontmatter (or a GHES API URL is otherwise detectable), the compiler should emit "GITHUB_HOST":"https://<ghes-host>" in the MCP server env block automatically. The value must include the https:// scheme — the MCP Server rejects bare hostnames since v0.30.3.
Suggested fix: Add a compiler pass that reads the workflow-level GH_HOST env var and, when set to a non-github.com value, injects GITHUB_HOST with the full URL (including scheme) into the MCP server container env.