fix(plugin): reject '/' as workspace, fall back to $HOME

[yannicklescure]

Summary

• resolveWorkspaceDirectory previously accepted any path it was handed, including a bare filesystem root (/). When opencode is launched without a real cwd (systemd unit, login shell at /, daemon spawned by root), worktree, directory, and process.cwd() can all resolve to /, which silently makes every tool operate against the whole machine and bypasses workspace scoping.
• Every candidate — env vars (CURSOR_ACP_WORKSPACE, OPENCODE_CURSOR_PROJECT_DIR), worktree, directory, and cwd — is now rejected if it resolves to / (or a bare Windows drive root like C:\).
• When no usable workspace is found we fall back to the current user's home directory before the OpenCode config prefix, so tools always land in a writable, user-scoped location instead of /.
• resolveWorkspaceDirectory and a new isRootPath helper are exported so the behaviour is unit-testable.

Why

The bundled ~/.config/opencode/plugin/cursor-acp.js was hot-patched in the field to work around this. This PR lands the same fix in the TypeScript source (src/plugin.ts) so a fresh bun run build produces a plugin that is safe by default.

Changes

• src/plugin.ts
  • Add isRootPath(pathValue) — detects / and bare Windows drive roots.
  • Add isAcceptableWorkspace(path, configPrefix) — rejects empty, root, and config-prefix paths in one place.
  • Reorder resolveWorkspaceDirectory so the configPrefix is computed once, each candidate goes through isAcceptableWorkspace, and a $HOME fallback is consulted before the config prefix.
  • Export resolveWorkspaceDirectory and isRootPath.
• tests/unit/plugin-workspace-resolution.test.ts (new)
  • isRootPath covers POSIX root, Windows drive roots (skipped off-Windows), and ordinary paths.
  • resolveWorkspaceDirectory covers: worktree preferred, directory fallback, / rejected from worktree/directory (falls back to cwd), / everywhere (falls back to \$HOME), env-var / rejection for both env vars, honoring a real env workspace, and skipping paths inside the config prefix.

Test plan

• bun test tests/unit/plugin-workspace-resolution.test.ts — 10 pass, 1 skipped (Windows-only case).
• bun test tests/unit/plugin.test.ts tests/unit/plugin-config.test.ts — no regressions.
• bun run build — bundles cleanly.
• Reviewer: verify on a real systemd-spawned daemon that the plugin no longer treats / as the workspace.

Note: tests/unit/plugin-tools-hook.test.ts > treats config path aliases (symlink/case variants) as config and falls back to workspace fails on main as well (unrelated symlink/tmp handling); not addressed here.

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