wait-for-hydra: add SSE support for real-time Hydra status

[angerman]

Summary

Add SSE (Server-Sent Events) support to the wait-for-hydra action for real-time build status from hydra-github-bridge. When hydra-status-url is provided, the action connects to the bridge's SSE endpoint instead of polling the GitHub API — reducing detection latency from minutes (poll interval) to seconds (event-driven).

Without hydra-status-url, the action behaves exactly as before (GitHub API polling with exponential backoff).

New input

Input	Default	Description
hydra-status-url	''	URL of the hydra-github-bridge (e.g. https://ci.zw3rk.com). Enables SSE mode when set.

How SSE mode works

1. One-shot check — queries the bridge's cached status endpoint (/status/{owner}/{repo}/{sha}) to see if the build already finished
2. SSE stream — connects to /status/{owner}/{repo}/{sha}/events and filters for events matching the requested check-run name
3. Periodic re-check — every 120s, re-queries the one-shot endpoint as a safety net against CDN buffering (e.g. Cloudflare) swallowing events
4. Timeout enforcement — read -t 60 prevents indefinite blocking on idle streams so the global timeout is always respected
5. Graceful fallback — if the SSE stream drops or times out without a terminal event, falls back to the existing poll loop

Other improvements in this PR

• Debug echo in poll_github() now shows the full jq query (copy-pasteable for local debugging) and goes to stderr so it doesn't contaminate conclusion=$(poll_github) capture
• Poll loop extracted into poll_wait() function for reuse as SSE fallback
• classify_status() helper centralizes success/failure classification

Usage

- uses: input-output-hk/actions/wait-for-hydra@latest
  with:
    check: required
    hydra-status-url: https://ci.zw3rk.com

Test plan

• Tested SSE one-shot path: returns immediately when build is already done
• Tested SSE stream path: receives events through Cloudflare CDN chain
• Tested poll fallback: works when SSE stream ends without terminal event
• Tested in production on devx PR #241 — passed in 6 seconds
• Verified backward compatibility: without hydra-status-url, action behaves identically to before