refactor!: drop the wiki and page layers

The plugin is issues and nothing else now. `skills/page` (the page-tree
domain) and `skills/wiki` (its bridge to a Gitea wiki) are gone, and
with them the issue domain's `wiki:` field — page titles were the only
thing that tied the two domains together, and a field the tracker has
no column for never came back from a pull anyway.

What is left is the shape AGENTS.md already claimed for the rest of the
repo: one domain, one bridge, one transport. The docs, the plugin
manifest, and tea-runner's skill table now say so too, and
test_payload_root walks the one script directory that remains.

Also removes openspec/config.yaml; nothing in the repo referenced it.

378 tests pass.

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

skills/issue/scripts/issue_new.py

@@ -155,8 +155,6 @@ 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()
@@ -182,7 +180,7 @@ 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, wiki=args.wiki)
+        depends=args.depends)
 
     # 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.