docs: add Gentoo installation to the README

[switch87]

Adds Gentoo to the Installation section of the README. On Gentoo the prebuilt binary is packaged as dev-util/opencode-bin in the ::guru overlay:

eselect repository enable guru
emerge dev-util/opencode-bin

Fixes #54313

[github-actions]

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[opencode-agent]

Thanks for adding the Gentoo install instructions, @switch87!

The dev branch (v1) only takes critical fixes now, so we can't merge README additions there. If the ::guru package also covers OpenCode v2 (@opencode/cli), you're welcome to open this against the v2 branch instead. Please check first that dev-util/opencode-bin installs the version that README describes.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

[switch87]

Thanks for pointing that out! Reopened against the v2 branch and using the PR template: #54316