fix: auto-resolve build artifact conflicts in milestone merge (#2777)

* fix: auto-resolve build artifact conflicts in milestone merge

The binary conflict classification in mergeMilestoneToMain only
auto-resolved .gsd/ prefixed files. Machine-generated build artifacts
like .tsbuildinfo, .pyc, __pycache__/, .DS_Store, and .map files were
treated as real code conflicts, blocking auto-merge unnecessarily.

Extract an isSafeToAutoResolve helper that checks both the .gsd/ prefix
and a SAFE_AUTO_RESOLVE_PATTERNS regex list. Matched files are resolved
with --theirs, same as .gsd/ state files.

Closes #2761

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* test: add tests for build artifact auto-resolve patterns

Extract isSafeToAutoResolve and SAFE_AUTO_RESOLVE_PATTERNS to module-level
exports for testability. Add unit tests covering .gsd/ state files, build
artifacts (.tsbuildinfo, .pyc, __pycache__, .DS_Store, .map), and rejection
of real source files (.ts, .js, .py, .json, .md).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/resources/extensions/gsd/auto-worktree.ts

@@ -157,6 +157,25 @@ function clearProjectRootStateFiles(basePath: string, milestoneId: string): void
   }
 }
 
+// ─── Build Artifact Auto-Resolve ─────────────────────────────────────────────
+
+/** Patterns for machine-generated build artifacts that can be safely
+ * auto-resolved by accepting --theirs during merge. These files are
+ * regenerable and never contain meaningful manual edits. */
+export const SAFE_AUTO_RESOLVE_PATTERNS: RegExp[] = [
+  /\.tsbuildinfo$/,
+  /\.pyc$/,
+  /\/__pycache__\//,
+  /\.DS_Store$/,
+  /\.map$/,
+];
+
+/** Returns true if the file path is safe to auto-resolve during merge.
+ * Covers `.gsd/` state files and common build artifacts. */
+export const isSafeToAutoResolve = (filePath: string): boolean =>
+  filePath.startsWith(".gsd/") ||
+  SAFE_AUTO_RESOLVE_PATTERNS.some((re) => re.test(filePath));
+
 // ─── Dispatch-Level Sync (project root ↔ worktree) ──────────────────────────
 
 /**
@@ -1408,30 +1427,30 @@ export function mergeMilestoneToMain(
         : nativeConflictFiles(originalBasePath_);
 
     if (conflictedFiles.length > 0) {
-      // Separate .gsd/ state file conflicts from real code conflicts.
-      // GSD state files (STATE.md, auto.lock, etc.)
-      // diverge between branches during normal operation — always prefer the
-      // milestone branch version since it has the latest execution state.
-      const gsdConflicts = conflictedFiles.filter((f) => f.startsWith(".gsd/"));
+      // Separate auto-resolvable conflicts (GSD state files + build artifacts)
+      // from real code conflicts. GSD state files diverge between branches
+      // during normal operation. Build artifacts are machine-generated and
+      // regenerable. Both are safe to accept from the milestone branch.
+      const autoResolvable = conflictedFiles.filter(isSafeToAutoResolve);
       const codeConflicts = conflictedFiles.filter(
-        (f) => !f.startsWith(".gsd/"),
+        (f) => !isSafeToAutoResolve(f),
       );
 
-      // Auto-resolve .gsd/ conflicts by accepting the milestone branch version
-      if (gsdConflicts.length > 0) {
-        for (const gsdFile of gsdConflicts) {
+      // Auto-resolve safe conflicts by accepting the milestone branch version
+      if (autoResolvable.length > 0) {
+        for (const safeFile of autoResolvable) {
           try {
-            nativeCheckoutTheirs(originalBasePath_, [gsdFile]);
-            nativeAddPaths(originalBasePath_, [gsdFile]);
+            nativeCheckoutTheirs(originalBasePath_, [safeFile]);
+            nativeAddPaths(originalBasePath_, [safeFile]);
           } catch {
             // If checkout --theirs fails, try removing the file from the merge
             // (it's a runtime file that shouldn't be committed anyway)
-            nativeRmForce(originalBasePath_, [gsdFile]);
+            nativeRmForce(originalBasePath_, [safeFile]);
           }
         }
       }
 
-      // If there are still non-.gsd conflicts, escalate
+      // If there are still real code conflicts, escalate
       if (codeConflicts.length > 0) {
         // Pop stash before throwing so local work is not lost (#2151).
         if (stashed) {

src/resources/extensions/gsd/tests/auto-worktree-auto-resolve.test.ts

@@ -0,0 +1,80 @@
+/**
+ * auto-worktree-auto-resolve.test.ts — Unit tests for isSafeToAutoResolve.
+ *
+ * Covers: .gsd/ state files, build artifacts (.tsbuildinfo, .pyc, __pycache__,
+ * .DS_Store, .map), and rejection of real source files.
+ */
+
+import { describe, test } from "node:test";
+import assert from "node:assert/strict";
+
+import {
+  isSafeToAutoResolve,
+  SAFE_AUTO_RESOLVE_PATTERNS,
+} from "../auto-worktree.ts";
+
+describe("isSafeToAutoResolve", () => {
+  // ─── .gsd/ state files ───────────────────────────────────────────────────
+  test("returns true for .gsd/ prefixed paths", () => {
+    assert.ok(isSafeToAutoResolve(".gsd/STATE.md"));
+    assert.ok(isSafeToAutoResolve(".gsd/milestones/M001/CONTEXT.md"));
+    assert.ok(isSafeToAutoResolve(".gsd/gsd.db"));
+  });
+
+  // ─── Build artifact patterns ─────────────────────────────────────────────
+  test("returns true for .tsbuildinfo files", () => {
+    assert.ok(isSafeToAutoResolve("tsconfig.tsbuildinfo"));
+    assert.ok(isSafeToAutoResolve("dist/tsconfig.tsbuildinfo"));
+  });
+
+  test("returns true for .pyc files", () => {
+    assert.ok(isSafeToAutoResolve("module.pyc"));
+    assert.ok(isSafeToAutoResolve("src/utils/helpers.pyc"));
+  });
+
+  test("returns true for __pycache__/ paths", () => {
+    assert.ok(isSafeToAutoResolve("src/__pycache__/module.cpython-311.pyc"));
+    assert.ok(isSafeToAutoResolve("lib/__pycache__/foo.py"));
+  });
+
+  test("returns true for .DS_Store files", () => {
+    assert.ok(isSafeToAutoResolve(".DS_Store"));
+    assert.ok(isSafeToAutoResolve("src/.DS_Store"));
+  });
+
+  test("returns true for .map source map files", () => {
+    assert.ok(isSafeToAutoResolve("dist/index.js.map"));
+    assert.ok(isSafeToAutoResolve("out/bundle.css.map"));
+  });
+
+  // ─── Real source files (should NOT be auto-resolved) ─────────────────────
+  test("returns false for .ts source files", () => {
+    assert.ok(!isSafeToAutoResolve("src/index.ts"));
+    assert.ok(!isSafeToAutoResolve("lib/utils.ts"));
+  });
+
+  test("returns false for .js source files", () => {
+    assert.ok(!isSafeToAutoResolve("src/index.js"));
+    assert.ok(!isSafeToAutoResolve("lib/helpers.js"));
+  });
+
+  test("returns false for .py source files", () => {
+    assert.ok(!isSafeToAutoResolve("src/main.py"));
+    assert.ok(!isSafeToAutoResolve("scripts/deploy.py"));
+  });
+
+  test("returns false for config and data files", () => {
+    assert.ok(!isSafeToAutoResolve("package.json"));
+    assert.ok(!isSafeToAutoResolve("tsconfig.json"));
+    assert.ok(!isSafeToAutoResolve("README.md"));
+  });
+
+  // ─── SAFE_AUTO_RESOLVE_PATTERNS export ────────────────────────────────────
+  test("SAFE_AUTO_RESOLVE_PATTERNS is a non-empty array of RegExp", () => {
+    assert.ok(Array.isArray(SAFE_AUTO_RESOLVE_PATTERNS));
+    assert.ok(SAFE_AUTO_RESOLVE_PATTERNS.length > 0);
+    for (const pattern of SAFE_AUTO_RESOLVE_PATTERNS) {
+      assert.ok(pattern instanceof RegExp);
+    }
+  });
+});