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.

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

README.md

@@ -445,6 +445,17 @@ src/devx/
 └── molecule/            # Optional molecule testing helpers (for Ansible projects)
 ```
 
+### Composite Actions (`.gitea/actions/`)
+
+Reusable Gitea composite actions for CI workflow steps:
+
+- `setup-env` — runs `make setup-image` (with optional `EXTRAS=`)
+- `notify-failure` — creates a Gitea issue on job failure
+- `quality-checks` — 6-step quality gate (lint, tests, speed, docs, translations, security)
+
+Each consumer repo gets its own copy (no cross-repo references). See
+ADR-0003 for the design rationale.
+
 ### Design principles
 
 - **Self-contained package** — `src/devx/` never imports from scripts outside the package