fix: use valid npm tag for v2 releases

[azumag]

Summary

• replace the invalid npm dist-tag v2 with opencode-v2
• bump the unpublished v2 package to 2.0.1 so the corrected publish workflow runs after merge
• update installation documentation

Cause

The first post-merge publish correctly avoided latest, but npm rejected v2 because it is a valid SemVer range. No 2.x package was published.

Validation

• npm run typecheck
• npm test -- __tests__/v2.test.ts