Replace VcsProcessExitError with CheckpointInvariantError for validation failures

VcsProcessExitError was being misused with fabricated exitCode values (0 and 1)
for application-level invariant violations: empty tree OID, empty commit OID,
and unavailable checkpoint refs. These are not process exit failures.

Use CheckpointInvariantError instead, which is already imported and is the
correct semantic fit for these validation-level conditions.

Author: cursoragent

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

@@ -14,7 +14,6 @@ import { randomUUID } from "node:crypto";
 import { Effect, Layer, FileSystem, Path } from "effect";
 
 import { CheckpointInvariantError } from "../Errors.ts";
-import { VcsProcessExitError } from "@t3tools/contracts";
 import { VcsDriver } from "../../vcs/VcsDriver.ts";
 import { CheckpointStore, type CheckpointStoreShape } from "../Services/CheckpointStore.ts";
 import { CheckpointRef } from "@t3tools/contracts";
@@ -128,11 +127,8 @@ const makeCheckpointStore = Effect.gen(function* () {
         });
         const treeOid = writeTreeResult.stdout.trim();
         if (treeOid.length === 0) {
-          return yield* new VcsProcessExitError({
+          return yield* new CheckpointInvariantError({
             operation,
-            command: "git write-tree",
-            cwd: input.cwd,
-            exitCode: 0,
             detail: "git write-tree returned an empty tree oid.",
           });
         }
@@ -146,11 +142,8 @@ const makeCheckpointStore = Effect.gen(function* () {
         });
         const commitOid = commitTreeResult.stdout.trim();
         if (commitOid.length === 0) {
-          return yield* new VcsProcessExitError({
+          return yield* new CheckpointInvariantError({
             operation,
-            command: "git commit-tree",
-            cwd: input.cwd,
-            exitCode: 0,
             detail: "git commit-tree returned an empty commit oid.",
           });
         }
@@ -234,11 +227,8 @@ const makeCheckpointStore = Effect.gen(function* () {
       }
 
       if (!fromCommitOid || !toCommitOid) {
-        return yield* new VcsProcessExitError({
+        return yield* new CheckpointInvariantError({
           operation,
-          command: "git diff",
-          cwd: input.cwd,
-          exitCode: 1,
           detail: "Checkpoint ref is unavailable for diff operation.",
         });
       }