feat(FN-4072): add diff volume gate to merger

Added a diff volume gate to the merger that blocks or warns on large diffs before merge completes. The feature includes a new `merger-diff-volume-gate` module wired into the merger, configurable via project settings, with tests covering all gate paths. Also updated docs and the changeset for the `@r

Fusion-Task-Id: FN-4072

packages/core/src/settings-schema.ts

@@ -211,6 +211,9 @@ export const DEFAULT_PROJECT_SETTINGS = {
   worktreeRebaseRemote: "",
   worktreeRebaseLocalBase: true,
   mergeConflictStrategy: "smart-prefer-main",
+  mergeDiffVolumeMinLines: undefined,
+  mergeDiffVolumeThreshold: undefined,
+  mergeDiffVolumeAllowlist: undefined,
   mergeStrategyOverlapBehavior: "flip-to-prefer-branch",
   workflowStepTimeoutMs: 360_000,
   workflowRevisionForkOnScopeMismatch: true,

packages/core/src/types.ts

@@ -2203,6 +2203,12 @@ export interface ProjectSettings {
   /** Strategy used when a merge conflict can't be resolved by AI. See
    *  {@link MergeConflictStrategy}. Default: "smart". */
   mergeConflictStrategy?: MergeConflictStrategy;
+  /** Minimum branch net line volume before the pre-commit diff-volume gate evaluates a file. Default applied at read site: 20. */
+  mergeDiffVolumeMinLines?: number;
+  /** Minimum staged/branch-net ratio required by the pre-commit diff-volume gate. Default applied at read site: 0.2. */
+  mergeDiffVolumeThreshold?: number;
+  /** Additional file globs allowlisted by the pre-commit diff-volume gate on top of generated/lockfile patterns. Default applied at read site: []. */
+  mergeDiffVolumeAllowlist?: string[];
   /** Controls overlap protection when `mergeConflictStrategy="smart-prefer-main"`
    *  reaches its Attempt 3 fallback. Default: "flip-to-prefer-branch". */
   mergeStrategyOverlapBehavior?: MergeStrategyOverlapBehavior;