fix(ci): resolve flaky api-command module resolution and migrate to self-hosted runner

Root cause: Bun's test runner fails to resolve exports from directory
index.ts files during parallel test execution. The api-command/index.ts
imports 8 subcommand modules — if any fail to load during concurrent
test runs, the entire module fails with "Export not found."

Fix: extract createApiCommandHandler into handler.ts (lightweight, only
imports named-command-router). Test imports from handler.ts directly,
bypassing the heavy index.ts that loads all subcommand modules.

Also:
- Remove stale api-command.ts barrel file
- Use explicit /index import paths in source
- Migrate CI validate to self-hosted runner with clean artifact step
- Disable Bun cache (no-cache: true) on self-hosted runner

.github/workflows/ci.yml

@@ -6,16 +6,20 @@ on:
 
 jobs:
   validate:
-    runs-on: ubuntu-latest
+    runs-on: [self-hosted, linux, x64]
 
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
 
+      - name: Clean stale artifacts
+        run: rm -rf node_modules ui/node_modules dist
+
       - name: Setup Bun
         uses: oven-sh/setup-bun@v2
         with:
           bun-version: '1.3.9'
+          no-cache: true
 
       - name: Setup Node.js
         uses: actions/setup-node@v4