Add GitHub Copilot as a built-in provider

apps/server/package.json

@@ -27,6 +27,8 @@
     "@effect/platform-node": "catalog:",
     "@effect/platform-node-shared": "catalog:",
     "@effect/sql-sqlite-bun": "catalog:",
+    "@github/copilot": "1.0.60",
+    "@github/copilot-sdk": "1.0.0",
     "@opencode-ai/sdk": "^1.3.15",
     "@pierre/diffs": "catalog:",
     "effect": "catalog:",

apps/server/src/checkpointing/Layers/CheckpointStore.test.ts

@@ -202,5 +202,43 @@ it.layer(TestLayer)("CheckpointStoreLive", (it) => {
         expect(whitespaceIgnoredDiff).not.toContain("+          <h1>Title</h1>");
       }),
     );
+
+    it.effect("returns a concise unavailable error when the target checkpoint ref is missing", () =>
+      Effect.gen(function* () {
+        const tmp = yield* makeTmpDir();
+        yield* initRepoWithCommit(tmp);
+        const checkpointStore = yield* CheckpointStore;
+        const threadId = ThreadId.make("thread-checkpoint-store-missing-target");
+        const fromCheckpointRef = checkpointRefForThreadTurn(threadId, 0);
+        const toCheckpointRef = checkpointRefForThreadTurn(threadId, 1);
+
+        yield* checkpointStore.captureCheckpoint({
+          cwd: tmp,
+          checkpointRef: fromCheckpointRef,
+        });
+        yield* writeTextFile(path.join(tmp, "README.md"), "changed\n");
+        yield* checkpointStore.captureCheckpoint({
+          cwd: tmp,
+          checkpointRef: toCheckpointRef,
+        });
+        yield* git(tmp, ["update-ref", "-d", toCheckpointRef]);
+
+        const error = yield* checkpointStore
+          .diffCheckpoints({
+            cwd: tmp,
+            fromCheckpointRef,
+            toCheckpointRef,
+            ignoreWhitespace: true,
+          })
+          .pipe(Effect.flip);
+
+        expect(error).toMatchObject({
+          _tag: "VcsProcessExitError",
+          command: "git diff",
+          detail: "Checkpoint ref is unavailable for diff operation.",
+        });
+        expect(error.message).not.toContain("ambiguous argument");
+      }),
+    );
   });
 });