The events page narrows to one kind: /:owner/:name/events?kind=pull.merged, and each row's kind links to it #8451

The events page now narrows to one kind with ?kind=, and each row's kind links to it. Gate, type check, design lint and test/web-events.test.ts (9 tests) all pass here. The branch is level with origin, so there was nothing new to push. I did not run the full suite; the forge runs that on push.

• Validation: /:owner/:name/events?kind=… checks kind with validEventsKind. An invalid kind gets the 404 page, not a 500.
• Row links: each row's kind is a link to ?kind=<that kind>, escaped wherever it is printed.
• Narrowed view: the heading names the kind and offers "All events". The next-page link keeps kind.
• Test: in test/web-events.test.ts, with a push and a merged pull, ?kind=push lists only the push and the cursor link carries kind.
• Beyond the issue: the kind buttons use person-worded labels. The terminal t key narrows to a kind, and this is documented.

To try it: open /:owner/:name/events?kind=pull.merged on the preview, then click a row's kind.

Closes #8430

The events page now narrows to one kind with ?kind=, and each row's kind links to it. Gate, type check, design lint and test/web-events.test.ts (9 tests) all pass here. The branch is level with origin, so there was nothing new to push. I did not run the full suite; the forge runs that on push.

• Validation: /:owner/:name/events?kind=… checks kind with validEventsKind. An invalid kind gets the 404 page, not a 500.
• Row links: each row's kind is a link to ?kind=<that kind>, escaped wherever it is printed.
• Narrowed view: the heading names the kind and offers "All events". The next-page link keeps kind.
• Test: in test/web-events.test.ts, with a push and a merged pull, ?kind=push lists only the push and the cursor link carries kind.
• Beyond the issue: the kind buttons use person-worded labels. The terminal t key narrows to a kind, and this is documented.

To try it: open /:owner/:name/events?kind=pull.merged on the preview, then click a row's kind.

Left #8430 open: 3 unticked items remain (?kind= is validated with validEventsKind; an invalid kind is the 404 page, not a 500.; Each row's kind is a link to ?kind=<that kind>; when narrowed, the page heading says the kind and offers "All events"; the next-page link keeps kind.; Test (test/web-events.test.ts): with a push and a merged pull, ?kind=push lists only the push; the cursor link carries kind.).