ci: add release-please automation with versioned branch workflow

- Add release-please config and manifest for Python releases
- Add release-please workflow triggered on release/** branches
- Add PyPI publish workflow on GitHub release
- Add helper workflow to cut release branches from main

Author: Jacksunwei

.github/.release-please-manifest.json

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+{
+  ".": "0.2.0"
+}

.github/release-please-config.json

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+{
+  "$schema": "https://raw.githubusercontent.com/googleapis/release-please/main/schemas/config.json",
+  "packages": {
+    ".": {
+      "release-type": "python",
+      "package-name": "google-adk-community",
+      "changelog-path": "CHANGELOG.md",
+      "extra-files": [
+        "src/google/adk_community/version.py"
+      ],
+      "changelog-sections": [
+        {"type": "feat", "section": "Features"},
+        {"type": "fix", "section": "Bug Fixes"},
+        {"type": "perf", "section": "Performance Improvements"},
+        {"type": "refactor", "section": "Code Refactoring"},
+        {"type": "docs", "section": "Documentation"},
+        {"type": "test", "section": "Tests", "hidden": true},
+        {"type": "build", "section": "Build System", "hidden": true},
+        {"type": "ci", "section": "CI/CD", "hidden": true},
+        {"type": "style", "section": "Styles", "hidden": true},
+        {"type": "chore", "section": "Miscellaneous Chores", "hidden": true}
+      ]
+    }
+  }
+}

.github/workflows/cut-release-branch.yml

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+name: Cut Release Branch
+
+on:
+  workflow_dispatch:
+    inputs:
+      version:
+        description: 'Version for the release branch (e.g., 0.3.0)'
+        required: true
+        type: string
+
+permissions:
+  contents: write
+
+jobs:
+  cut-branch:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          ref: main
+          fetch-depth: 0
+
+      - name: Validate version format
+        run: |
+          if ! echo "${{ inputs.version }}" | grep -qE '^[0-9]+\.[0-9]+\.[0-9]+$'; then
+            echo "Error: Version must be in format X.Y.Z (e.g., 0.3.0)"
+            exit 1
+          fi
+
+      - name: Create release branch
+        run: |
+          BRANCH_NAME="release/v${{ inputs.version }}"
+          git checkout -b "$BRANCH_NAME"
+          git push origin "$BRANCH_NAME"
+          echo "Created and pushed branch: $BRANCH_NAME"
+          echo "Release-please will now create a release PR on this branch."

.github/workflows/publish-pypi.yml

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+name: Publish to PyPI
+
+on:
+  release:
+    types: [published]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  publish:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install uv
+        uses: astral-sh/setup-uv@v4
+        with:
+          version: "latest"
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+
+      - name: Build package
+        run: uv build
+
+      - name: Publish to PyPI
+        env:
+          UV_PUBLISH_TOKEN: ${{ secrets.PYPI_TOKEN }}
+        run: uv publish

.github/workflows/release-please.yml

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+name: Release Please
+
+on:
+  push:
+    branches:
+      - 'release/**'
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  release-please:
+    runs-on: ubuntu-latest
+    outputs:
+      release_created: ${{ steps.release.outputs.release_created }}
+      tag_name: ${{ steps.release.outputs.tag_name }}
+    steps:
+      - uses: googleapis/release-please-action@v4
+        id: release
+        with:
+          config-file: .github/release-please-config.json
+          manifest-file: .github/.release-please-manifest.json
+          target-branch: ${{ github.ref_name }}