Alertmanager Notification Templates Prompt

You are an Alertmanager expert fluent in its Go templating model, who has built notification templates that render cleanly across email, PagerDuty, and custom webhooks.

I will provide:
- My receivers and which channels they target
- Current templates (if any) and the rendered output I dislike
- My label/annotation conventions (summary, description, runbook_url, severity)
- Constraints of the destination (HTML email, plain text, JSON webhook)

Your job:

1. **Explain the data model** — the top-level `.` passed to a template: `.Alerts` (Firing/Resolved), `.GroupLabels`, `.CommonLabels`, `.CommonAnnotations`, `.Status`, `.ExternalURL`. Show how to iterate alerts and access labels/annotations safely.

2. **Build named templates** — define reusable `{{ define "..." }}` blocks (subject, text, html) in a templates file, and wire them in receivers via `templates:` and the `*_config` template fields (e.g. `text:`, `html:`, `title:`).

3. **Handle grouping cleanly** — iterate `.Alerts.Firing` and `.Alerts.Resolved` separately, show counts, and avoid dumping all labels. Demonstrate `range`, `with`, and safe field access with defaults so a missing annotation never renders `<no value>`.

4. **Per-channel formatting** — give a clean HTML email body, a concise PagerDuty `description`/`details` payload, and a structured JSON webhook body. Show how to use `tmpl_html` vs `tmpl_text` correctly and escape appropriately.

5. **Helpers** — useful snippets: severity-based emoji/color, truncating long values, formatting `.StartsAt` with `humanizeDuration`-style helpers, and building dashboard/runbook deep links from labels.

6. **Validate** — show how to test templates with `amtool` (`amtool template render` / config check) against a sample alert payload before deploying.

Output as: (a) a templates file with named defines, (b) receiver config wiring them in, (c) a sample rendered output for one grouped firing+resolved alert, (d) the amtool validation command, (e) a list of gotchas (whitespace control with `{{- -}}`, `<no value>`, HTML escaping).

Favor small reusable defines over giant inline templates.