fix(website): cross-locale doc links + drop empty ko locale (#31895)

The locale switcher appeared broken because hardcoded markdown links
(`](/docs/X)`) got double-prefixed by Docusaurus to `/docs/<locale>/docs/X`
(404) in non-English locales, and the MDX hero `<a href>` on the index
page escaped locale routing entirely.

Changes:
- Rewrite 922 `](/docs/X)` -> `](/X)` across 166 docs files (strip trailing
  .md too). Docusaurus prepends locale + baseUrl itself.
- docs/index.md -> index.mdx; hero "Get Started" anchor -> Docusaurus
  <Link> so it stays inside the active locale.
- Drop `ko` locale entirely from docusaurus.config.ts + delete i18n/ko/
  (4 stale auto-translated kanban pages, <2% coverage, misleading).

Verified `npm run build` succeeds for both en and zh-Hans; `build/zh-Hans/
index.html` has no /docs/zh-Hans/docs/... double-prefixed paths.

PR2 will translate the 335 English docs into i18n/zh-Hans/.

website/docs/user-guide/features/delegation.md

@@ -197,7 +197,7 @@ The TUI ships a `/agents` overlay (alias `/tasks`) that turns recursive `delegat
 - Kill and pause controls — cancel a specific subagent mid-flight without interrupting its siblings
 - Post-hoc review: step through each subagent's turn-by-turn history even after they've returned to the parent
 
-The classic CLI just prints `/agents` as a text summary; the TUI is where the overlay shines. See [TUI — Slash commands](/docs/user-guide/tui#slash-commands).
+The classic CLI just prints `/agents` as a text summary; the TUI is where the overlay shines. See [TUI — Slash commands](/user-guide/tui#slash-commands).
 
 ## Depth Limit and Nested Orchestration