feat(FN-4068): add branch conflict detection and recovery for stale worktre

Implements branch conflict detection and recovery across the Fusion engine, CLI, and dashboard — surfacing git worktree conflicts when tasks conflict with unrelated branch state, and providing a recovery workflow to resolve them. The executor and worktree pool now integrate typed branch conflict che

Fusion-Task-Id: FN-4068

packages/core/src/__tests__/settings-parity.test.ts

@@ -73,6 +73,12 @@ describe("settings key parity", () => {
     expect(DEFAULT_PROJECT_SETTINGS.heartbeatMultiplier).toBe(1);
   });
 
+  it("defaults sibling branch rename escape hatch to disabled", () => {
+    expect(DEFAULT_PROJECT_SETTINGS.executorAllowSiblingBranchRename).toBe(false);
+    expect(isProjectSettingsKey("executorAllowSiblingBranchRename")).toBe(true);
+    expect(isGlobalSettingsKey("executorAllowSiblingBranchRename")).toBe(false);
+  });
+
   it("defaults completionDocumentationMode to off", () => {
     expect(DEFAULT_PROJECT_SETTINGS.completionDocumentationMode).toBe("off");
   });

packages/core/src/settings-schema.ts

@@ -180,6 +180,7 @@ export const DEFAULT_PROJECT_SETTINGS = {
   testCommand: undefined,
   buildCommand: undefined,
   recycleWorktrees: false,
+  executorAllowSiblingBranchRename: false,
   worktreeNaming: "random",
   taskPrefix: "FN",
   includeTaskIdInCommit: true,

packages/core/src/types.ts

@@ -2081,6 +2081,10 @@ export interface ProjectSettings {
    *  of being deleted. New tasks acquire a warm worktree from the pool,
    *  preserving build caches (node_modules, target/, dist/). Default: false. */
   recycleWorktrees?: boolean;
+  /** When true, restores the legacy behavior of silently creating sibling
+   *  branches like `fusion/FN-123-2` when the canonical task branch is already
+   *  checked out elsewhere. Default: false. */
+  executorAllowSiblingBranchRename?: boolean;
   /** Controls how worktree directory names are generated when creating fresh worktrees.
    *  Only applies when recycleWorktrees is NOT enabled (pooled worktrees retain their existing names).
    *  - "random": Human-friendly adjective-noun names (e.g., swift-falcon) — default