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/skills/bundled/software-development/software-development-plan.md

@@ -21,7 +21,7 @@ Plan mode: write markdown plan to .hermes/plans/, no exec.
 | License | MIT |
 | Platforms | linux, macos, windows |
 | Tags | `planning`, `plan-mode`, `implementation`, `workflow` |
-| Related skills | [`writing-plans`](/docs/user-guide/skills/bundled/software-development/software-development-writing-plans), [`subagent-driven-development`](/docs/user-guide/skills/bundled/software-development/software-development-subagent-driven-development) |
+| Related skills | [`writing-plans`](/user-guide/skills/bundled/software-development/software-development-writing-plans), [`subagent-driven-development`](/user-guide/skills/bundled/software-development/software-development-subagent-driven-development) |
 
 ## Reference: full SKILL.md