fix: report the real API rate-limit budget from response headers

[JustMaris]

github_rate_limit_remaining sat at 5000/5000 almost permanently: over 7 days it dipped only once, to 4962. That's because GET /rate_limit reports used: 0 for our tokens. The headers on ordinary API calls showed the real count at the same time, e.g. used: 1210. As a result, GithubApiRateLimitLow could never fire.

What GitHub is doing

According to GitHub's REST rate-limit docs:

• Requests are processed in several regions, so "rate limit values can vary from one response to the next".
• The x-ratelimit-* headers are authoritative over /rate_limit.

Measured with one token, it's counted in two core windows at once, each with its own reset time and count:

Endpoints	Window reset	Remaining
/orgs/{o}/actions/runners, /repos/{r}/actions/workflows, /repos/{r}/dependabot/alerts	17:43 UTC	~3,495
everything else tried (/user, /orgs/{o}, repos, pulls, issues, commits, /actions/runs, contents)	18:12 UTC	~4,865

Changes

• The client records the X-RateLimit-Limit/Remaining/Reset headers of every core response and keeps one entry per window (keyed by reset time). Within a window it keeps the lowest count seen. Windows drop off once they reset.
• github_rate_limit_remaining / _limit now report the scarcest active window, the one that runs out first, so the alert and dashboard need no changes. They're read at scrape time from the client, so the 15s runner poll keeps them fresh. They were previously part of the 5-minute org snapshot.
• New github_rate_limit_window_remaining{reset}: one series per active window.
• The dashboard's rate-limit-over-time panel also plots each window.
• The /rate_limit call is gone, which saves one request per org refresh.

Verification

• go test -race ./... passes. The new client test covers two concurrent windows, a non-core resource being ignored, and window expiry.
• Local run against drumandbytes: github_rate_limit_window_remaining{reset="17:43:05Z"} 3367 and {reset="18:12:54Z"} 4797. Live headers moments later read 3365 and 4796, and github_rate_limit_remaining = 3367.