Configure the temporary root for Parallel forks

Problem

Parallel creates an isolated copy of the workspace for every branch, but the fork staging location is harness-owned and currently lands under the OS temporary directory. There is no operator configuration or environment hook to select another filesystem.

This blocks otherwise read-only parallel investigation when /tmp is space-constrained even though the workspace filesystem has ample capacity. In the observed case, three read-only branches each attempted to copy the repository including the ignored website/node_modules tree, and creation failed with no space left on device under /tmp.

Desired outcome

Provide an operator-configurable temporary root for Parallel fork workspaces. The configured root should be used for fork creation and cleanup, while the current OS-temp behavior remains the default when unset.

Acceptance criteria

• An operator can configure the Parallel fork temporary root without changing a workspace repository.
• The setting is applied to every Parallel branch, including read-only investigation branches.
• The default remains the current OS temporary-directory behavior.
• Invalid, unwritable, or insufficient-space configured roots fail clearly before or during fork creation; no silent fallback to an unexpected filesystem.
• Fork cleanup uses the configured root and leaves no orphaned directories after normal completion.
• Documentation identifies the setting and distinguishes it from mecatui XDG_RUNTIME_DIR or TMPDIR, which only controls mecatui embedded-server socket placement.

Context

The existing Parallel tool API has no destination/root parameter. This should be an operator deployment setting rather than a model-controlled per-call argument.