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.

.github/PULL_REQUEST_TEMPLATE.md

@@ -6,6 +6,17 @@
     Types in use: feat, fix, docs, test, chore, perf, refactor, style.
     See CONTRIBUTING.md § Commit Conventions.
 
+  Still writing code? Prefix the title with `WIP:` —
+    WIP: feat(media): add play-media fzf picker
+  Gitea recognizes the prefix, marks the PR a draft, and blocks merging
+  until you remove it. The prefix goes before the whole conventional
+  subject, and comes off when the branch is complete.
+
+  `WIP:` means MORE CHANGES ARE COMING. It is not for a finished branch
+  waiting on verification — that is what an unchecked box in Verification
+  already signals. A complete branch with open checkboxes is a normal PR,
+  not a WIP.
+
   Keep the two headings below, in this order, and delete these comments.
   Optional sections may be added between them (see the note above
   "Verification").