fix(runner): update GITHUB_TOKEN for gh CLI after credential refresh

[ambient-code]

Summary

• Fixes GITHUB_TOKEN env var not updated after credential refresh, breaking all gh CLI operations #1135 — gh CLI operations fail with 401 after mid-session credential refresh because the subprocess's GITHUB_TOKEN env var is stale
• Adds a gh CLI wrapper script (/tmp/bin/gh) that reads the fresh token from /tmp/.ambient_github_token and exports GH_TOKEN before exec-ing the real gh binary
• Follows the same pattern as the existing git credential helper — token files are already updated on every refresh, so the wrapper just reads them at invocation time
• The wrapper is installed by populate_runtime_credentials() and prepended to PATH so it intercepts all gh invocations in the CLI subprocess

How it works

The CLI subprocess environment is fixed at spawn time. When refresh_credentials runs mid-session, os.environ["GITHUB_TOKEN"] is updated in the runner process but NOT in the subprocess. The git credential helper already solved this for git by reading from a token file. This PR applies the same solution for gh:

1. install_gh_wrapper() writes a shell script to /tmp/bin/gh
2. The script reads /tmp/.ambient_github_token (already written on every refresh)
3. If a fresh token is found, it exports GH_TOKEN (which takes precedence over GITHUB_TOKEN)
4. It then finds and execs the real gh binary, skipping its own directory

Test plan

• test_install_creates_executable_wrapper — verifies the wrapper is created and executable
• test_install_prepends_to_path — verifies /tmp/bin is prepended to PATH
• test_install_is_idempotent — verifies repeated calls don't duplicate PATH entries
• test_populate_installs_gh_wrapper — verifies populate_runtime_credentials() installs the wrapper
• All 32 existing credential tests continue to pass

🤖 Generated with Claude Code