docs: document cross-repository automation

docs/cross-repository-automation.md

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+<!--
+SPDX-FileCopyrightText: 2026 LibreCode coop and contributors
+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
+
+# Cross-repository automation
+
+LibreCode cross-repository workflow automation is authenticated through the
+**LibreCode Workflow Automation** GitHub App.
+
+## Installation
+
+The App is installed on the `LibreCodeCoop` organization with access to all
+repositories.
+
+The installation-level access is intentionally broad so newly created LibreCode
+repositories do not require a manual App reconfiguration before they can be
+onboarded.
+
+Actual automation remains opt-in:
+
+- catalog publication targets only `LibreCodeCoop/.github`;
+- consumer synchronization targets only repositories declared in
+  `consumers.json`;
+- catalog entries are limited by `workflow-catalog.json`.
+
+## GitHub App permissions
+
+Repository permissions:
+
+- Contents: read/write;
+- Pull requests: read/write;
+- Workflows: read/write;
+- Metadata: read.
+
+No organization administration, members, secrets or repository administration
+permissions are required.
+
+## Repository configuration
+
+`LibreCodeCoop/github-workflows` stores:
+
+- Actions variable `LIBRECODE_WORKFLOW_APP_ID`;
+- Actions secret `LIBRECODE_WORKFLOW_APP_PRIVATE_KEY`.
+
+The private key must never be committed to the repository.
+
+## Token model
+
+Workflows do not store a long-lived installation token.
+
+Each write-capable job uses `actions/create-github-app-token`, pinned to a
+full commit SHA, to create a short-lived installation token.
+
+Although the App is installed across the organization, each generated token is
+further restricted to the exact destination repository:
+
+- catalog publisher: `.github`;
+- consumer sync: the current consumer repository from the matrix.
+
+The token also requests only the permissions needed for the operation.
+
+## Onboarding a repository
+
+A new repository does not require reinstalling or reconfiguring the App.
+
+To opt a repository into managed workflow synchronization:
+
+1. validate the desired workflows in that repository;
+2. add the repository and workflow names to `consumers.json`;
+3. merge the reviewed change;
+4. review the automatically created adoption PR;
+5. merge the lock file;
+6. confirm a subsequent synchronization is a no-op.
+
+## Publishing a template
+
+A generated file under `workflow-templates/` is not automatically public.
+
+Add the template name to `workflow-catalog.json` only after the workflow has
+completed its security and consumer validation.
+
+This prevents release, credential-sensitive or experimental workflows from
+appearing in **Actions -> New workflow** prematurely.
+
+## Private-key rotation
+
+Rotate the App private key when:
+
+- compromise is suspected;
+- an administrator with access to the key leaves the responsible team;
+- organizational security policy requires rotation.
+
+Rotation procedure:
+
+1. generate a new private key in the GitHub App settings;
+2. replace `LIBRECODE_WORKFLOW_APP_PRIVATE_KEY` in the repository Actions
+   secrets;
+3. run/observe catalog publication and consumer synchronization successfully;
+4. delete the old private key from the GitHub App settings.
+
+Do not delete the old key before the new key has been validated.
+
+## Validation
+
+The initial production validation confirmed:
+
+- GitHub App configuration can be read by Actions;
+- scoped installation tokens can be created;
+- the catalog repository can be checked out and updated;
+- a catalog no-op does not leave an update PR open;
+- `LibreCodeCoop/extract` can be checked out and updated;
+- managed workflows can be adopted into the lock file;
+- a subsequent synchronization with current hashes creates no PR.