feat: implement the wiki field in the issue domain

format.md listed `wiki:` among the domain fields, between depends and
origin, and issue.py had never heard of it. The field fell into extra and
rendered with the foreign keys — sorted in after the sync fields, which
the same document forbids one line below the table. Written without
brackets it parsed as a single string, and nothing but a text editor
could set it.

Implemented rather than de-documented: page_ls.py --titles already
prints these titles, so the field was designed and only unwired.
DOMAIN_KEYS and LIST_KEYS learn it, Issue carries it, and issue_new.py
gets a repeatable --wiki flag.

Titles only, as the format says: no path, no sub_url, no lookup. The
tracker has no field for it, so it is never sent and a pull does not
bring it back — format.md now says so.

Closes #32

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

skills/issue/scripts/issue_new.py

@@ -155,6 +155,8 @@ def main():
     ap.add_argument("--assignee", action="append", default=[], help="assignee; repeat")
     ap.add_argument("--depends", action="append", default=[],
                     help="id this issue depends on; repeat")
+    ap.add_argument("--wiki", action="append", default=[],
+                    help="page title this work is written up in; repeat")
     ap.add_argument("--out", default=issue.ISSUE_ROOT,
                     help="store root (default: <repo>/tmp/issues)")
     args = ap.parse_args()
@@ -180,7 +182,7 @@ def main():
         id=id, title=args.title,
         body=with_depends(TEMPLATES[args.type], args.depends),
         labels=labels, assignees=args.assignee, milestone=args.milestone,
-        depends=args.depends)
+        depends=args.depends, wiki=args.wiki)
 
     # The first issue in a fresh checkout has to create the store, but it says
     # so — and it says where, because the path is absolute.