fix(cli): reject stale platform binaries during install

[thdxr]

Issue for this PR

Closes #

Type of change

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

What does this PR do?

opencode upgrade could report success while leaving the previous release installed. When a platform package version (e.g. @opencode/cli-linux-x64@0.0.0-dev-20719) was not yet visible on npm, Bun skipped the optional dependency, kept the old platform package directory, and exited 0. The postinstall script then picked that package by name, saw its binary run, and linked the old binary under the new launcher.

The postinstall script now rejects a platform package whose version does not match the launcher's optionalDependencies, so it falls through to the next candidate (for example the baseline build) or to its npm fallback. If nothing matches, it exits 1, the package manager fails, and opencode upgrade reports the failure.

How did you verify your code works?

• On a copy of a real install stuck in this state (launcher 20719, cli-linux-x64 20718): the old script kept 20718; the patched script skipped the stale package and linked cli-linux-x64-baseline@0.0.0-dev-20719, and the binary reported 20719.
• With no matching platform package, the script exits 1 with the existing "Failed to install OpenCode" message.
• Confirmed bun install --global --trust exits 1 when a postinstall script fails.
• bun run check passes.

Screenshots / recordings

N/A

Checklist

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

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