docs: add DCO requirement and PR template

- CONTRIBUTING.md: add Developer Certificate of Origin section, license
  policy for contributions, requirement to include SPDX headers on new
  source files.
- .github/workflows/dco.yml: enforce Signed-off-by trailer on every PR
  commit using tim-actions/get-pr-commits + tim-actions/dco.
- .github/PULL_REQUEST_TEMPLATE.md: PR template with DCO sign-off
  checkbox, test plan, and change-type categories.

Context: spent ~30min wiring up the standard OSS contributor workflow —
DCO is the lightweight alternative to a CLA, recognized by the Linux
Foundation, CNCF, and most Apache 2.0 projects.

Signed-off-by: tazhate <hate@tazhate.ru>

Author: tazhate

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,36 @@
+<!--
+Thank you for contributing to chainplane!
+Please fill out the sections below and remove any that do not apply.
+-->
+
+## Summary
+
+<!-- A short description of what this PR does and why. -->
+
+## Type of change
+
+- [ ] Bug fix (non-breaking change which fixes an issue)
+- [ ] New feature (non-breaking change which adds functionality)
+- [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
+- [ ] Documentation update
+- [ ] New chain adapter
+- [ ] CI / build / tooling
+
+## Testing
+
+<!-- How was this verified? `make test`, `make test-e2e`, manual kind cluster, etc. -->
+
+- [ ] `make test` passes
+- [ ] `make lint` passes
+- [ ] `make manifests generate` produces no diff
+
+## DCO sign-off
+
+By submitting this PR I certify that:
+
+- [ ] My commits are signed off (`git commit -s`) and comply with the [DCO](https://developercertificate.org/).
+- [ ] I am the original author of the contribution, or have the right to submit it under Apache 2.0.
+
+## Related issues
+
+<!-- Closes #N, refs #M -->

.github/workflows/dco.yml

@@ -0,0 +1,25 @@
+name: DCO
+
+on:
+  pull_request:
+    types: [opened, synchronize, reopened]
+
+permissions:
+  contents: read
+  pull-requests: read
+
+jobs:
+  dco:
+    name: DCO check
+    runs-on: ubuntu-latest
+    steps:
+      - name: Get PR commits
+        id: get-commits
+        uses: tim-actions/get-pr-commits@v1.3.1
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Verify Signed-off-by trailer
+        uses: tim-actions/dco@v1.1.0
+        with:
+          commits: ${{ steps.get-commits.outputs.commits }}

CONTRIBUTING.md

@@ -2,6 +2,22 @@
 
 Thank you for your interest in contributing. This document covers the development workflow, testing, and how to add support for new blockchain chains.
 
+## Developer Certificate of Origin (DCO)
+
+All contributions to chainplane must be made under the [Developer Certificate of Origin](https://developercertificate.org/). By signing off your commits you certify that:
+
+- The contribution was created by you and you have the right to submit it under the project license; or
+- The contribution is based on previous work licensed under the same open-source license; or
+- The contribution was provided to you by a third party who has the right to submit it under the project license.
+
+**How to sign off:** add `Signed-off-by: Your Name <you@example.com>` to each commit. The `git commit -s` flag does this automatically.
+
+A DCO check runs on every pull request. PRs without sign-off cannot be merged.
+
+## License of Contributions
+
+All contributions are accepted under the [Apache License, Version 2.0](LICENSE). New source files must include the SPDX header from `hack/boilerplate.go.txt`.
+
 ## Prerequisites
 
 - **Go 1.23+** (version pinned in `go.mod`)