OpenCode contribution and vertical-tabs PR workflow

OpenCode requires a linked issue for every PR; scan for duplicates and prefer an open, maintainer-assigned issue. PRs must target anomalyco/opencode:dev from a fork and follow .github/pull_request_template.md; missing sections can trigger auto-close after two hours. Use concise prose, Closes #..., verification details, and screenshots where relevant. Keep each commit PR-ready and cherry-pick only the requested subset. Because issue #36942 followed backlash against horizontal tabs, frame vertical tabs as opt-in rather than a default replacement. PR #38308 used this workflow: one squashed commit, linked issue, standalone typecheck/build, and promised screenshots.