Add DSH to the agent roster and Install > AI

Official npm @deepseek-ai/dsh (bin: dsh) rides the mise path like Dim.
dsh has no TUI, so the interactive launch is the documented 'dsh web'
and one-shot prompts use 'dsh --profile headless'. Auth is dsh-managed,
so it stays out of the AI Hub registry.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019hLK3wsDuKVAC37GgDqg6H

test/shell.d/menu-test.sh

@@ -248,7 +248,7 @@ assertDeepEqual(
   defaultItems
     .filter(item => item.parent === 'setup.default.agent')
     .map(item => item.label),
-  ['Antigravity', 'Claude', 'Codex', 'Copilot', 'Crush', 'Deep Code', 'Dim Agent', 'Grok', 'Kimi Code', 'omp', 'OpenCode', 'Ori', 'Pi', '国产模型组合 (AI Hub)'],
+  ['Antigravity', 'Claude', 'Codex', 'Copilot', 'Crush', 'Deep Code', 'Dim Agent', 'DSH', 'Grok', 'Kimi Code', 'omp', 'OpenCode', 'Ori', 'Pi', '国产模型组合 (AI Hub)'],
   'menu sorts coding agents alphabetically'
 )
 const expectedDefaults = {