Migrate off deprecated @octokit/webhooks-types

`@octokit/webhooks-types` is deprecated ("Package no longer supported") and unmaintained since 2024-10. `@octokit/webhooks@14.2.0` now depends on `@octokit/openapi-webhooks-types` instead, but that package has no drop-in equivalent for the friendly union types we use (`WebhookEvent`, `PullRequestEvent`, `PushEvent`, `WorkflowDispatchEvent`, `WorkflowRunEvent` in `src/router.ts`, `src/autoupdater.ts`, `src/types.ts`) — it's raw OpenAPI-generated output (`operations["pull-request/opened"]["requestBody"]...`), so migrating means hand-building those unions per event action. Deferred for now since it still works; revisit when there's time to verify the reconstructed types carefully against real payloads.

Start by reading the current type imports and uses in src/router.ts, src/autoupdater.ts, and src/types.ts, then compare them with the raw output from @octokit/openapi-webhooks-types. Reconstruct the event unions only after checking their action shapes against real webhook payloads; done means the deprecated dependency is removed without breaking the existing event typing.

Written by the indexing model from the issue text.