docs: fix pnpm v2 install command

[opencode-agent]

Summary

• put pnpm’s --allow-build option before the add command, matching pnpm’s documented invocation
• keep the @next tag explicit so the V2 CLI is installed

Verification

• installed into an isolated pnpm home and confirmed opencode2 v0.0.0-next-17041
• blume check
• blume validate
• website production build

Requested by: @kitlangton (Kit Langton via X)

[opencode-agent]

Closing this: both the existing and reordered pnpm commands install the current @next build in a clean environment, so this change does not address the reported stale tui-v2 installation. The root cause needs a reproducible diagnosis first.

[VanTanev]

Wrong, this is reproducable; pnpm v11 rejects @next tag because it's less than 24 hours old.

Actual fix is:

pnpm add -g --config.minimum-release-age=0 --allow-build=@opencode-ai/cli @opencode-ai/cli@next