Add gh uptime to report GitHub service status - #14094

What

Adds gh uptime: a command that reports the operational status of GitHub's
services from the public status page (https://www.githubstatus.com).

$ gh uptime
✓ All Systems Operational

  ✓ Git Operations         operational
  ✓ API Requests           operational
  ✓ Actions                operational
  ...

It queries the Statuspage summary API — not api.github.com — and sends no
authentication, so it works when your token is unset, and (the point) when the
API itself is degraded. The gh auth token is never sent to the third-party host.

Why

Partly for fun, but there's a real use case: holding automation until an
incident clears, then resuming. During an Actions outage you currently have to
poll githubstatus.com by hand. With this you can let a script wait it out:

# wait out an Actions outage, then kick off a workflow
gh uptime --component Actions --watch && gh workflow run ci.yml

Flags aimed at scripting:

• --json / --jq — machine-readable output (gh uptime -c Actions --jq .operational → a bare bool)
• --component <name> — scope to a single service (e.g. Actions)
• --exit-code — non-zero exit when the target is not operational, so it composes with &&
• --watch / --interval — poll until the target returns to operational (min 5s to respect the status page)

Notes

• No new dependencies; uses the existing cmdutil/iostreams/httpmock machinery.
• Auth is disabled for the command (DisableAuthCheck) since the status page is public.
• Non-service rows the status page carries (e.g. the "Visit www.githubstatus.com…" link) are filtered out.
• Unit tests cover flag parsing, all-operational vs degraded rendering, --exit-code, component scoping, unknown component, and --json export — all via httpmock, no network.

Happy to adjust naming/scope — I picked uptime to avoid colliding with the existing gh status (notifications/activity).

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