feat(FN-4950): reinforce recycleWorktrees opt-in default copy

Fusion-Task-Id: FN-4950
Fusion-Task-Lineage: ce37aa18-a5ff-4aef-a93b-768edd07978c

.changeset/fn-4950-recycle-worktrees-default-off.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": patch
+---
+
+Lock in default: `recycleWorktrees` is off by default (opt-in). Regression tests now guard the invariant, and dashboard/docs copy explicitly states the default.

docs/settings-reference.md

@@ -263,7 +263,7 @@ Sandbox backend precedence is:
 | `worktreeInitCommand` | `string` | `undefined` | Shell command run after worktree creation. For pnpm repos, prefer `pnpm install --frozen-lockfile` for deterministic bootstrap. |
 | `testCommand` | `string` | `undefined` | Merge-time test command (hard gate). When unset, Fusion auto-detects from lockfile. |
 | `buildCommand` | `string` | `undefined` | Merge-time build command (hard gate). |
-| `recycleWorktrees` | `boolean` | `false` | Reuse worktrees from a pool for faster startup. |
+| `recycleWorktrees` | `boolean` | `false` | Default: off (opt-in). Reuse worktrees from a pool for faster startup. |
 | `executorAllowSiblingBranchRename` | `boolean` | `false` | Opt back into the legacy executor behavior that silently allocates sibling branches (`fusion/<task-id>-2`, `-2-2`, …) when the canonical task branch is already checked out elsewhere. When disabled (default), branch conflicts fail loudly, leave the task in `todo` with `status: "failed"`, and expose stranded commits for explicit recovery via [`fn task branch-recovery`](./cli-reference.md#fn-task). See [Task Management → Branch conflict recovery](./task-management.md#branch-conflict-recovery). The dashboard Settings modal exposes the same toggle with warning copy because this legacy mode is discouraged. |
 | `worktreeNaming` | `"random" \| "task-id" \| "task-title"` | `"random"` | Naming mode for new worktree directories. |
 

packages/dashboard/app/components/SettingsModal.tsx

@@ -4163,7 +4163,7 @@ export function SettingsModal({
                 />
                 Recycle worktrees
               </label>
-              <small>When enabled, completed task worktrees are returned to an idle pool instead of being deleted, preserving build caches for faster startup</small>
+              <small>Off by default (opt-in). When enabled, completed task worktrees are returned to an idle pool instead of being deleted, preserving build caches for faster startup</small>
             </div>
             <div className="form-group">
               <label htmlFor="executorAllowSiblingBranchRename" className="checkbox-label">