docs: add ADR-0003 and update docs for composite actions

New ADR-0003 documenting composite action design rationale and Gitea
1.27 constraints. Updated AGENTS.md, README.md, architecture.md, and
ci-cd-workflow.md with composite action documentation. CHANGELOG entry.

Generated with [Devin](https://devin.ai)

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

docs/tech/architecture.md

@@ -453,6 +453,24 @@ v2 failures. Supports loading custom platforms from a JSON file.
 - **Secrets via environment** — secrets are passed via environment variables,
   never on the command line.
 
+## Composite Actions (`.gitea/actions/`)
+
+Reusable Gitea composite actions eliminate repeated multi-step sequences
+across workflows. Each action lives in `.gitea/actions/<name>/action.yml`
+and is referenced via `uses: ./.gitea/actions/<name>`.
+
+| Action | Purpose |
+|--------|---------|
+| `setup-env` | Run `make setup-image` (with optional `EXTRAS=`) |
+| `notify-failure` | Create a Gitea issue on job failure via `devx.ci.notify_failure` |
+| `quality-checks` | 6-step quality sequence: lint, tests, speed, docs, translations, security |
+
+Each consumer repo (`devx`, `grm`, `infra`) gets its own copy — there are
+no cross-repo composite action references. This avoids a single-point-of-failure
+where a bad devx master commit would break all repos' CI simultaneously.
+
+See ADR-0003 for the full design rationale and Gitea 1.27 constraints.
+
 ## Import rules
 
 1. **`src/devx/` is self-contained** — the package never imports from outside `src/`