docs(contributing): document the WIP: title prefix for in-progress PRs

Gitea treats a `WIP:` title prefix as a draft marker and refuses to merge
while it's present. Verified empirically against this instance by
temporarily retitling PR #123: the API flipped `draft` to true and
`mergeable` to false, and both reverted when the prefix was removed.

The distinction from the verification merge gate is the point, so it's
stated explicitly in both files: `WIP:` means more code changes are
coming, NOT "finished but unverified". A complete branch with unticked
`## Verification` boxes is an ordinary PR — that case is already covered
by the merge gate and does not want the prefix. The two signals are
independent and can occur in any combination.

Documented as a merge rule under Branching & Pull Requests, next to the
verification gate, and in the template's title guidance where an author
picking a title will actually encounter it.

CONTRIBUTING.md

@@ -51,6 +51,16 @@ assumes you *do* have push access (maintainers, regular contributors).
 - **Don't merge until the `## Verification` checklist is fully checked.**
   Unchecked boxes are outstanding manual checks, not decoration. See
   [Pull request descriptions](#pull-request-descriptions) below.
+- **Prefix in-progress PRs with `WIP:`.** If the branch still has code
+  changes coming, open it as `WIP: type(scope): description`. Gitea
+  recognizes the prefix, flags the PR as a draft, and refuses to merge it
+  until the prefix is removed; drop it once the branch is complete.
+
+  `WIP:` signals **more changes are coming** — not "done but unverified".
+  A finished branch waiting on manual checks is an ordinary PR whose
+  `## Verification` boxes aren't all ticked yet; that's already the merge
+  gate above and doesn't need the prefix. The two are independent: a PR
+  can be WIP with everything ticked, or complete with checks outstanding.
 - **Keep feature branches focused.** If you stumble onto something unrelated
   to your current task while working (a pre-existing bug, a stray cleanup),
   don't fold it into the same commit or PR. Handle it with one of these,