fix(completion): sync top-level command suggestions

[inori-3333]

Description

Fix shell completion so top-level command suggestions match the current CLI surface.
Btw, sorry for my last PR which didn't attach the test output. This time I atttached it below, thanks.

This change:

• removes stale setup from completion candidates
• adds missing top-level built-ins: plugin, install, and register
• adds a unit test covering the top-level completion list to prevent regressions

Related issue:

• N/A

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🌐 New site adapter
• 📝 Documentation
• ♻️ Refactor
• 🔧 CI / build / tooling

Checklist

• I ran the checks relevant to this PR
• I updated tests or docs if needed
• I included output or screenshots when useful

Documentation (if adding/modifying an adapter)

N/A — this PR does not add or modify an adapter.

• Added doc page under docs/adapters/ (if new adapter)
• Updated docs/adapters/index.md table (if new adapter)
• Updated sidebar in docs/.vitepress/config.mts (if new adapter)
• Updated README.md / README.zh-CN.md when command discoverability changed
• Used positional args for the command's primary subject unless a named flag is clearly better
• Normalized expected adapter failures to CliError subclasses instead of raw Error

Screenshots / Output

Checks run:

• npx vitest run src/completion.test.ts --project unit
• npm run typecheck

Completion output after this change:

E:\Inori_Code\Intrest\fromGit\opencli> npx vitest run src/completion.test.ts --project unit

 RUN  v4.1.1 E:/Inori_Code/Intrest/fromGit/opencli

 ✓  unit  src/completion.test.ts (2 tests) 4ms
   ✓ getCompletions (2)
     ✓ includes top-level built-ins that are registered outside the site registry 2ms
     ✓ still includes discovered site names 0ms

 Test Files  1 passed (1)
      Tests  2 passed (2)
   Start at  01:38:35
   Duration  239ms (transform 37ms, setup 0ms, import 82ms, tests 4ms, environment 0ms)

E:\Inori_Code\Intrest\fromGit\opencli> npm run typecheck

> @jackwener/opencli@1.5.5 typecheck
> tsc --noEmit