Refactor/deduplicate print and graphql headers

[dataCenter430]

Summary

Extend the deduplication from #482 — eliminate three remaining instances of the same patterns.

• Add _print(message, json_mode) helper to miner_commands/helpers.py, replacing three bare if not json_mode: console.print(...) blocks across post.py (2 sites) and check.py (1 site)
• Add make_graphql_headers(token) factory to github_api_tools.py next to make_headers, replacing two inline {'Authorization': f'Bearer {token}', 'Content-Type': 'application/json'} dicts at execute_graphql_query and get_github_graphql_query
• Replace hardcoded timeout=30 in get_github_user() with GITHUB_HTTP_TIMEOUT_SECONDS — the one call site missed by refactor: centralize GitHub API constants and deduplicate console.status pattern #482

Note: the timeout change is a behaviour change (30 s → 15 s per attempt); get_github_user has a 6-attempt retry loop so the tightening is intentional.