feat: link the dashboard's PR and Dependabot panels through to GitHub

[JustMaris]

Drill-down from the dashboard to the actual lists on github.com, plus two panel fixes from the live dashboard.

Exporter

• New github_org_info{org} (always 1). The dashboard reads it through a hidden org variable to build links, so imports work for any org without a hand-set variable.

Dashboard

• Open PRs by Repo:
  • clicking a bar opens github.com/<org>/<repo>/pulls;
  • the panel link opens the org-wide open-PR search;
  • repo names sit above the bars with fixed sizes. Before, one bar filled the panel and the name was clipped to "g…".
• New "Dependabot Alerts by Repo" table: one row per repo, critical/high/medium/low columns, 0 for empty cells. Each repo links to its Dependabot alerts.
• Dependabot Alerts by Severity: shows "No open alerts" instead of "No data", and its panel link opens the org's Dependabot overview.
• Open PRs (Org-wide) stat: links to the org-wide PR search.
• Layout: the three panels share the Organization row, 8 columns each.

Verification

• go test ./... passes.
• Rendered in a local Grafana 13.2.2 against the cluster Prometheus. With fake alerts loaded into the local copy only, the table showed momentarr 0/3/1, eraser 2/0/0.
• Links checked in the rendered page:
  • github.com/drumandbytes/{momentarr,eraser}/security/dependabot
  • github.com/drumandbytes/github-actions-runner-exporter/pulls
  • github.com/pulls?q=is:open+is:pr+org:drumandbytes
• Until 1.4.0 is deployed, github_org_info doesn't exist and the links have an empty org.