Add Dim Agent CLI to the agent roster and Install > AI

npm dimcode (bin: dim) rides the mise path like Deep Code; entries
land alphabetically in the Default Agent and Install > AI menus.
CLI-only per dimagent.cn, login handled by the CLI itself.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QKxGW1raAWaqeU8WdHsMsp

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', 'Grok', 'Kimi Code', 'omp', 'OpenCode', 'Ori', 'Pi', '国产模型组合 (AI Hub)'],
+  ['Antigravity', 'Claude', 'Codex', 'Copilot', 'Crush', 'Deep Code', 'Dim Agent', 'Grok', 'Kimi Code', 'omp', 'OpenCode', 'Ori', 'Pi', '国产模型组合 (AI Hub)'],
   'menu sorts coding agents alphabetically'
 )
 const expectedDefaults = {