Add Buildkite webhook provider

[VeldtJumper]

Summary

• add a /buildkite provider for all documented Buildkite Pipelines events, Package Registries package.created, and the documented Test Engine workflow.alarm envelope
• validate matching body/header event discriminators, render bounded Discord-safe embeds, and only link trusted Buildkite HTTPS hosts
• package a canonical example and register Buildkite in provider discovery/example delivery
• add focused coverage, README setup and security notes, the website provider tile, and Simple Icons attribution

Verification

• npm test — 162 passed
• npm run lint
• npm run build
• (cd web && npm test && npm run build)
• production Docker image build and smoke test, including packaged examples, provider discovery, and a 200 readiness response
• headless browser visual check of the provider grid and Buildkite logo

Security note

Buildkite supports plaintext-token and raw-body HMAC authentication. Skyhook does not store the Buildkite webhook secret, so this provider cannot authenticate either form; the README documents that limitation. Incoming text is treated as untrusted display data, Discord mentions are suppressed, and embed links are restricted to HTTPS Buildkite hosts.