ci: add workflow_dispatch with branch and prerelease inputs (#10)

## Summary

- Add `workflow_dispatch` trigger with `branch` and `prerelease` inputs
- Pass `target-branch` and `prerelease` to the release-please action
- Push to main uses config default (`prerelease: true`), dispatch can
override

## Test plan

- [ ] Trigger workflow manually from Actions tab, verify inputs appear
- [ ] Push to main, verify release-please still creates alpha
pre-release PRs
- [ ] Dispatch with `prerelease: false` for a stable release

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

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

Author: FBumann

.github/workflows/release.yaml

@@ -3,6 +3,16 @@ name: Release
 on:
   push:
     branches: [main]
+  workflow_dispatch:
+    inputs:
+      branch:
+        description: Branch to create the release from
+        required: true
+        default: main
+      prerelease:
+        description: Create a pre-release
+        type: boolean
+        default: true
 
 permissions:
   contents: write
@@ -28,6 +38,8 @@ jobs:
           config-file: .release-please-config.json
           manifest-file: .release-please-manifest.json
           token: ${{ steps.app-token.outputs.token }}
+          target-branch: ${{ inputs.branch || github.ref_name }}
+          prerelease: ${{ inputs.prerelease }}
 
   publish:
     name: Build & publish to PyPI