platform: unify the discord alert format across bugsink, ci, and axiom

[zgeoff]

Three separate paths post alerts to the alarms Discord channel, each with its own ad-hoc format:

1. Bugsink — new-issue, regression, and unmute alerts through the stock Discord messaging-service webhook (apps/bugsink/README.md § Alerts).
2. CI — the alert job in .github/workflows/main.yml posts a hand-rolled embed when the main pipeline goes red.
3. Axiom monitors — OTel metric monitors fire through the "vers alarms (Discord)" notifier, in Axiom's stock format.

None of them share a shape, so the channel is hard to scan and alert kind/severity doesn't stand out.

Scope

• Define one alert format for the channel: source, severity/kind, what fired, the one link that opens the right investigation view, and a color scheme consistent across sources.
• Per path, decide how close each can get: native template config where the tool supports it, a small relay reformatting the webhook payload where it doesn't, and bring the CI embed in line directly in the workflow.
• Whatever ships updates apps/bugsink/README.md § Alerts, the new-project steps in docs/architecture/deployment.md, and wherever the Axiom notifier setup is recorded.

The Bugsink webhook credential is the bugsink-discord-webhook item in the vers 1Password vault; CI uses the DISCORD_WEBHOOK_URL secret.

[zgeoff]

Closed via #695, which unified the one source with a configurable body — CI now posts the canonical structured embed. Axiom and Bugsink stay their stock formats: Bugsink exposes no templating, and Axiom's custom-webhook notifier is plan-gated (400 feature not enabled: notifiersCustomWebhook). Axiom adoption once that feature is enabled is parked in #697 with the full implementation.