A webhook that fails three deliveries in a row shows one line in its repository writers' feed #10161

For: a broken hook is silent today; redeliver and pause exist (docs/features/webhook-redeliver.md, webhook-pause.md) but nobody is told to use them.

Done looks like:

• [x] On the third failed delivery in a row (src/core/webhooks.ts) one event goes on the bus, once per streak, not per failure.
• [ ] Writers of the repository see one line: "Webhook to example.com failed 3 times: <reason>", linking to Settings › Webhooks; readers do not.
• [x] A success ends the streak; a fourth failure adds no second line.
• [x] Tests for once-per-streak, writers-only, escaping of the host and reason.

Reviewed pull #10170 at its head: ticked On the third failed delivery in a row (src/core/webhooks.ts) one event goes on the bus, once per streak, not per failure.; A success ends the streak; a fourth failure adds no second line.; Tests for once-per-streak, writers-only, escaping of the host and reason.. left open: Writers of the repository see one line: "Webhook to example.com failed 3 times: <reason>", linking to Settings › Webhooks; readers do not. — The line links to the repository and only names Settings › Webhooks in text, because Settings is admin-only. This is a deliberate trade-off, so the box stays open..

Left open after pull #10170 landed: 1 unticked item remains (Writers of the repository see one line: "Webhook to example.com failed 3 times: <reason>", linking to Settings › Webhooks; readers do not.). The planner files each as its own issue and closes this one; a person may close it, tick what is done, or say what is left. Until then the crew does not pick it again.

Split: the streak event and the writers-only line shipped. What is left of the open box is the link to Settings › Webhooks, and what a writer who is not an admin sees there: #10196.