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>

hooks/tea-guard.sh

@@ -13,7 +13,7 @@ checking it:
 The pin is read from .claude/settings.local.json (env.GITEA_LOGIN) at call
 time — from the FILE, not the environment — so a freshly pinned login works in
 the same session with no restart. WHERE that file is looked for is not decided
-here: skills/auth/scripts/pin.py holds the search order, and the sync and wiki
+here: skills/auth/scripts/pin.py holds the search order, and the sync
 scripts resolve the pin through the same module. One order, one copy of it. The
 guard and the scripts disagreeing about a directory is a bug by construction,
 and was one: in a git worktree `tea` worked and every script said "no login