feat(cli): support Vite+ installations

[msmps]

Issue for this PR

Closes #49623

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Vite+ keeps global packages in its own store, so npm and pnpm cannot detect or manage an OpenCode installation created with vp.

This adds vp as an installation method, detects exact package matches from Vite+'s JSON output, uses Vite+ for updates and removal, and handles package-name migration with a forced install. It also documents Vite+ installation and explicit upgrades.

How did you verify your code works?

• Ran the complete CLI suite: 287 tests passed.
• Ran bun run check from the repository root.
• Ran the website typecheck, generated-file check, and production build.
• Tested an isolated VP_HOME: detection, no-match handling, removal, an exact update to @opencode/cli@2.0.7, opencode --version, and forced package-name migration all passed.

Screenshots / recordings

OpenCode installed in an isolated VP_HOME with Vite+ detects vp through /update, upgrades from 2.0.6 to 2.0.7, and reports success.

vite-plus-update-demo.mp4

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR