fix: add release-please v4 configuration files

Add required configuration for release-please v4:
- .release-please-manifest.json: Tracks current version (1.0.0)
- release-please-config.json: Configures release strategy for Python package

Without these files, release-please v4 cannot properly detect and bump
versions for breaking changes. This will allow the breaking change in
commit 7559751 to properly trigger a major version bump to 2.0.0.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: semarj

.release-please-manifest.json

@@ -0,0 +1,3 @@
+{
+  ".": "1.0.0"
+}

release-please-config.json

@@ -0,0 +1,11 @@
+{
+  "packages": {
+    ".": {
+      "release-type": "python",
+      "package-name": "shipengine",
+      "changelog-path": "CHANGELOG.md",
+      "bump-minor-pre-major": false,
+      "bump-patch-for-minor-pre-major": false
+    }
+  }
+}