Add missing lockfiles to skip list and show fallback for empty stderr

Several ecosystem lockfiles (Nix flake, Gradle, npm-shrinkwrap, Swift PM)
were missing from SKIP_RESOLVE, causing unnecessary conflict resolution
attempts. Empty stderr on failure produced confusing error messages.

src/commands/helpers.ts

@@ -96,6 +96,10 @@ const SKIP_RESOLVE = new Set([
   "Podfile.lock",
   "poetry.lock",
   "pubspec.lock",
+  "flake.lock",
+  "gradle.lockfile",
+  "npm-shrinkwrap.json",
+  "Package.resolved",
   "uv.lock",
   "yarn.lock",
 ])
@@ -124,7 +128,8 @@ export async function resolveConflicts(
     )
 
     if (resolved.exitCode !== 0) {
-      throw new Error(`Claude failed to resolve ${file}: ${resolved.stderr.toString().trim()}`)
+      const stderr = resolved.stderr.toString().trim()
+      throw new Error(`Claude failed to resolve ${file}: ${stderr || "(no output)"}`)
     }
 
     await Bun.write(join(cwd, file), resolved.stdout.trimEnd() + "\n")

src/git.ts

@@ -137,7 +137,8 @@ export async function commit(message: string, cwd: string): Promise<void> {
 export async function checkoutTheirs(file: string, cwd: string): Promise<void> {
   const result = await $`git checkout --theirs -- ${file}`.cwd(cwd).nothrow().quiet()
   if (result.exitCode !== 0) {
-    throw new Error(`Failed to checkout theirs for ${file}: ${result.stderr.toString().trim()}`)
+    const stderr = result.stderr.toString().trim()
+    throw new Error(`Failed to checkout theirs for ${file}: ${stderr || "(no output)"}`)
   }
 }
 
@@ -145,7 +146,8 @@ export async function checkoutTheirs(file: string, cwd: string): Promise<void> {
 export async function stageFile(file: string, cwd: string): Promise<void> {
   const result = await $`git add ${file}`.cwd(cwd).nothrow().quiet()
   if (result.exitCode !== 0) {
-    throw new Error(`Failed to stage ${file}: ${result.stderr.toString().trim()}`)
+    const stderr = result.stderr.toString().trim()
+    throw new Error(`Failed to stage ${file}: ${stderr || "(no output)"}`)
   }
 }