docs: restructure DCO guidance into scannable subsection (#3337)

Extracts the dense inline DCO paragraph from the "Before You Open a PR"
section into a dedicated `### Sign Your Commits` subsection.

## What changed

- Adds a `### Sign Your Commits` heading directly below the Conventional
Commits paragraph
- Leads with the command (`git commit -s`) in a code block
- Follows with a plain-English explainer of what the sign-off does
- Adds linkable `#### Fix unsigned commits already pushed` and `####
Auto-setup for future commits` subheadings
- Removes the old inline paragraph (content preserved, structure only
changed)

## Why

The existing guidance was buried mid-paragraph; contributors may not
find it until CI blocks them. This makes the requirement and its fix
immediately visible and actionable.

## Notes

Docs-only change, no code modified.

Signed-off-by: Cameron Hotchkies <chotchkies@block.xyz>
Co-authored-by: npub1ep9tf72jk6xgwamqj5m2j0xvqvwm9vdu3zxlz7cesxg53x52tkkqf6pa42 <c84ab4f952b68c8777609536a93ccc031db2b1bc888df17b198191489a8a5dac@buzz.block.builderlab.xyz>

CONTRIBUTING.md

@@ -43,7 +43,28 @@ Buzz is an agent platform, so AI-assisted PRs are welcome. No need to disclose t
 
 We squash-merge, so your PR title becomes the commit subject in `main`. Use [Conventional Commits](https://www.conventionalcommits.org/) format: `feat(mcp): add get_feed_actions tool`. The type prefix (`feat`, `fix`, `docs`, `refactor`, `test`, `chore`) is required. See the [Commit Messages](#commit-messages) section for the full reference.
 
-Every commit needs a Developer Certificate of Origin sign-off, so commit with `git commit -s` — it appends the `Signed-off-by` trailer that certifies you wrote the change and can contribute it. The required **DCO Check** blocks merge without it on every commit, and it's the most common reason new PRs stall. If you already pushed unsigned commits, run `git rebase --signoff main` and force-push. Running `just hooks` installs a `commit-msg` hook that adds the trailer to commits created by `git commit` and `git merge`; other flows need their own flag — `git rebase --signoff`, `git cherry-pick -s`.
+### Sign Your Commits
+
+```bash
+git commit -s
+```
+
+Every commit needs a Developer Certificate of Origin (DCO) sign-off. The `-s` flag appends a `Signed-off-by` trailer that certifies you wrote the change and can contribute it under the project license. The **DCO Check** will block your PR without it.
+
+#### Fix unsigned commits already pushed
+
+```bash
+git rebase --signoff main
+git push --force-with-lease
+```
+
+#### Auto-setup for future commits
+
+```bash
+just hooks
+```
+
+This installs a `commit-msg` hook that adds the sign-off trailer automatically for `git commit` and `git merge`. Other flows (`git rebase`, `git cherry-pick`) still need their own flag — `--signoff` and `-s` respectively.
 
 We review as capacity allows — focused PRs that follow this guide move fastest.