feat(FN-5329): remove orphan rescue and branch-recovery primitives from eng

Removes the branch-recovery CLI surface, orphan-rescue engine primitives, and their associated tests (over 1,500 lines deleted), while restoring a minimal prune-only orphan branch sweep with proper git audit mutation types. Documentation across `cli-reference.md`, `task-management.md`, and `AGENTS.m

Fusion-Task-Id: FN-5329

packages/engine/src/worktree-pool.ts

@@ -830,25 +830,14 @@ export async function reapOrphanWorktrees(
   return removed;
 }
 
-/** Columns where the merger handles branch cleanup — skip these during orphan scanning. */
+/** Columns where merger/finalization owns branch lifecycle. */
 const MERGER_MANAGED_COLUMNS: ReadonlySet<Column> = new Set(["in-review", "done"]);
 
 /**
- * Scan for orphaned `fusion/*` branches that are not associated with any
- * non-archived, non-merger-managed task.
- *
- * Lists all local branches matching the `fusion/*` pattern, then compares
- * against branches stored on tasks (via `task.branch` or derived as
- * canonicalFusionBranchName(taskId)). Branches belonging to tasks in the
- * `in-review` or `done` columns are excluded because the merger is
- * responsible for cleaning those up.
- *
- * @param rootDir — Project root directory (git working tree)
- * @param store — Task store for listing tasks and their branch assignments
- * @returns Array of orphaned branch names
+ * Return local `fusion/*` branches not associated with any active task.
+ * Branches tied to merger-managed or archived tasks are excluded.
  */
 export async function scanOrphanedBranches(rootDir: string, store: TaskStore): Promise<string[]> {
-  // List all local branches matching fusion/*
   let allBranches: string[];
   try {
     const result = await execAsync("git branch --list 'fusion/*'", {
@@ -868,23 +857,14 @@ export async function scanOrphanedBranches(rootDir: string, store: TaskStore): P
 
   if (allBranches.length === 0) return [];
 
-  // Build set of branches associated with active (non-archived, non-merger-managed) tasks
   const tasks = await store.listTasks({ slim: true, includeArchived: false });
   const activeBranches = new Set<string>();
   for (const task of tasks) {
-    // Skip tasks in columns where the merger handles branch cleanup
     if (MERGER_MANAGED_COLUMNS.has(task.column)) continue;
-    // Also skip archived tasks
     if (task.column === "archived") continue;
-
-    // Use stored branch name if available, otherwise derive from task ID
-    if (task.branch) {
-      activeBranches.add(task.branch);
-    }
-    // Always add the derived name too — the task may not have `branch` set yet
+    if (task.branch) activeBranches.add(task.branch);
     activeBranches.add(canonicalFusionBranchName(task.id));
   }
 
-  // Return branches not associated with any active task
   return allBranches.filter((branch) => !activeBranches.has(branch));
 }