ci: add composite actions for setup-env, notify-failure, quality-checks

Three Gitea composite actions in .gitea/actions/ that eliminate repeated
multi-step sequences across workflows. Each consumer repo gets its own
copy (no cross-repo references). See ADR-0003 for design rationale.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

.gitea/actions/setup-env/action.yml

@@ -0,0 +1,37 @@
+name: 'Set up environment'
+description: 'Set up CI environment with venv and PATH (calls make setup-image)'
+
+# Composite action for the common "Set up environment" step pattern.
+# Replaces the repeated inline:
+#   - name: Set up environment
+#     env:
+#       CI_GITEA_API_TOKEN: ${{ secrets.CI_GITEA_API_TOKEN }}
+#     run: make setup-image
+#
+# Gitea 1.27 notes:
+# - Every `run` step needs explicit `shell:`.
+# - Composite actions cannot access `secrets` directly; they read from
+#   the `env:` context which the calling workflow must populate.
+# - The calling workflow's top-level `env:` block (CI_GITEA_API_TOKEN,
+#   CI_GITEA_USERNAME) is visible here via `${{ env.* }}`.
+
+inputs:
+  extras:
+    description: 'Extra pip install groups passed to make setup-image (e.g., ci,lint,release)'
+    required: false
+    default: ''
+
+runs:
+  using: 'composite'
+  steps:
+    - name: Set up environment
+      shell: bash
+      env:
+        CI_GITEA_API_TOKEN: ${{ env.CI_GITEA_API_TOKEN }}
+        CI_GITEA_USERNAME: ${{ env.CI_GITEA_USERNAME }}
+      run: |
+        if [ -n "${{ inputs.extras }}" ]; then
+          make setup-image EXTRAS="${{ inputs.extras }}"
+        else
+          make setup-image
+        fi