Add /meta endpoint support to gh CLI proxy #5923

Problem

The gh CLI probes the /meta endpoint (/api/v3/meta) during initialization for configuration and feature detection. When workflows use gh commands in pre-agent steps that route through the mcpg DIFC proxy, the proxy returns 403 Forbidden because it does not handle /meta:

HTTP 403: 403 Forbidden (https://localhost:18443/api/v3/meta)

This caused the Issue Arborist workflow to fail: https://github.com/github/gh-aw/actions/runs/25803527731

The workaround (gh-aw PR #31954) was to replace gh issue list with a direct REST API call, but this is fragile — any workflow using gh CLI commands through the proxy will hit the same issue.

Proposed Solution

Add support for the /meta endpoint in the mcpg proxy. The proxy should either:

1. Forward /meta requests to the upstream GitHub API, or
2. Return a minimal valid response that satisfies the gh CLI initialization check

Option 1 is preferred for correctness; option 2 is acceptable if there are security/DIFC concerns with forwarding.

References

• Failed run: https://github.com/github/gh-aw/actions/runs/25803527731
• Workaround PR: Fix Issue Arborist pre-agent issue fetch gh-aw#31954
• gh CLI /meta probe: gh calls GET /meta on startup to detect GitHub instance capabilities