docs: add a privacy page for the GitHub App

docs/privacy.md

@@ -0,0 +1,62 @@
+# Privacy
+
+This page describes what the **Commit Check GitHub App** reads, what it
+keeps, and what it never sees. The command-line tool, the pre-commit hooks
+and the GitHub Action run entirely inside your own environment and send
+nothing anywhere; this page is about the hosted App only.
+
+## What the App reads
+
+When you install the App, GitHub grants it these permissions and nothing
+else:
+
+| Permission | Level | Used for |
+|---|---|---|
+| Metadata | read | Knowing which repositories it is installed on |
+| Contents | read | Fetching the commits under test and the repository's `cchk.toml` / `commit-check.toml` |
+| Checks | write | Posting the **Commit Check** result on each commit |
+| Pull requests | read | Listing the commits of a pull request |
+
+On every push and pull request event the App fetches the commits under
+test into a temporary directory. The fetch is shallow and filtered
+(`--filter=blob:none`), so what comes across the wire is the commit
+metadata — message, author, timestamps, the list of paths each commit
+touches — plus the contents of exactly one file: the configuration file,
+if the repository has one. **The contents of no other file are ever
+downloaded**, not even into the temporary directory's object store. The
+rules are evaluated, the result is posted as a check run, and the temporary
+directory is deleted before the event is considered handled.
+
+## What the App keeps
+
+Nothing. The App has no database and stores no repository content, commit
+messages, author details or configuration. Each event is processed from the
+webhook payload and discarded.
+
+The hosting platform retains **operational logs** for a short period. A log
+line carries the webhook delivery id, the commit SHA being checked, the id
+of the check run created and, when a check could not run, the error text
+the tool produced — never an author name or email, and never file content.
+
+## What the App never sees
+
+- Your source code beyond the configuration file named above
+- Repositories the App is not installed on
+- The contents of a delivery whose signature does not verify. GitHub signs
+  the raw request body, so the App has to receive that body to check the
+  signature; an unsigned or mis-signed delivery is rejected at that point,
+  before anything in it is parsed or acted on
+
+## Where it runs
+
+The App runs on [Fly.io](https://fly.io) in the `ams` (Amsterdam) region.
+Traffic between GitHub and the App is encrypted in transit; GitHub signs
+every delivery and the App verifies the signature before doing anything
+else.
+
+## Contact
+
+Questions about this page or the App's data handling:
+[open an issue](https://github.com/commit-check/commit-check/issues).
+
+_Last updated: 2026-09-03._

mkdocs.yml

@@ -163,5 +163,6 @@ nav:
       - Rules: rules.md
       - Configuration: configuration.md
       - Changelog: changelog.md
+      - Privacy: privacy.md
   - Blog:
       - blog/index.md