feat(cli): port the upgrade command to v2

[opencode-agent]

Summary

Add the missing V2 upgrade [target] command, backed by the existing CLI updater rather than the legacy installation stack.

• Resolve the latest version from the current build's release channel, or install an explicit version with an optional leading v.
• Detect the installation method automatically and support --method / -m overrides for curl, npm, pnpm, bun, and yarn.
• Skip the already installed version, reject invalid targets, and report lookup/install failures with a nonzero exit code.
• Preserve the V2 installer URL, package-manager build/trust flags, and temporary-cache cleanup. Correct Node-distribution detection and installation to use the published opencode-node package rather than its opencode2-node executable name.

The command does not start or restart the background service. Explicit upgrades do not depend on the automatic-update policy. Unknown installation methods require an explicit override; V1-only package-manager integrations are not advertised.

Usage

opencode2 upgrade
opencode2 upgrade <version>
opencode2 upgrade <version> --method bun

Validation

• CLI package: bun test test/upgrade.test.ts test/updater-install.test.ts src/services/updater.test.ts test/debug-paths.test.ts test/cors.test.ts — 47 passed.
• CLI package: bun typecheck — passed.
• Prettier and git diff --check — passed.
• Scoped Oxlint — zero errors; five consistent-return warnings.
• Installer tests inject process results and use isolated temporary directories; no real installations or network downloads were performed.

Requested by: @rekram1-node (Aiden via Slack)