[thesiti92]

Why

Every commit or PR board created two detached worktrees of the repository under .git/dev-fast/reviews/<uuid>/: the head and the base. For a large monorepo each one is a full tree on disk. Since #1175 the base side of a diff has no language features, so the base checkout served only Source windows opened from the base side of a diff.

What

• ReviewWorkspaces prepares one environment per board, the head. open() is gone; source(reviewId, pins) has no side. Rows and worktrees with the legacy role: "base" are collected on the next start, so existing users reclaim the disk without doing anything. The head/<sha> path segment stays so existing head checkouts survive the upgrade.
• A base-side "open in Source window" opens the head checkout and the file's head counterpart; a file that no longer exists at head answers 404 as before. The base content stays in the Diff and in code peeks, read from git objects.
• environmentIssues (review_open, review_environment) is {message}[]. The language-context and navigator routes drop the side query; the fork no longer sends it.
• ensureReviewPinnedCheckout and reviewManagedCheckoutDir lose their role parameter.

Tests

• New: a legacy base row plus worktree is removed on startup while the head checkout and row stay.
• Updated the workspaces, local-data, review-api, headless-host, gateway and head-checkout suites for head-only environments.
• pnpm lint, pnpm format:check, review package typecheck and the fork typecheck pass. src/cli-install.test.ts "runs a matched CLI and runtime with healthy discovery" fails identically on an unchanged origin/main worktree on this machine.

Follow-ups, not in this PR: create the head checkout on demand instead of on open, and point diffr at the git dir (verified: identical tree-mode output from a checkout, its .git dir and a bare clone).