feat(spaces): docs-kind spaces render as pure documentation repos

Adds a `kind` column to spaces ('project' default, 'docs' for Wiki).
Docs spaces skip projects/tasks fetches and render only the page tree.
Sidebar caret for docs spaces expands to top-level pages (#/page/:id).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/db/migrations/021_space_kind.sql

@@ -0,0 +1,5 @@
+-- 021: space kind — 'project' (workspace w/ projects+tasks) or 'docs' (pure documentation repo)
+ALTER TABLE spaces ADD COLUMN IF NOT EXISTS kind text NOT NULL DEFAULT 'project';
+ALTER TABLE spaces DROP CONSTRAINT IF EXISTS spaces_kind_check;
+ALTER TABLE spaces ADD CONSTRAINT spaces_kind_check CHECK (kind IN ('project','docs'));
+UPDATE spaces SET kind='docs' WHERE slug='wiki';