doc: add branching and PR rules to collaboration guide

All changes must go through a feature branch and PR for review
before merging to main.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

COLLABORATING.md

@@ -56,6 +56,30 @@ Examples:
 - `needs-discussion` — decision needed before work starts
 - `wontfix` — acknowledged, not addressing
 
+## Branching and Pull Requests
+
+All changes go through a branch and a pull request — never commit directly to `main`.
+
+### Branch naming
+
+```
+<type>/<short-description>
+```
+
+Examples:
+- `feat/received-documents-person-page`
+- `fix/tag-filter-url-sync`
+- `devops/docker-compose-v2`
+
+### Workflow
+
+1. Create a branch from `main` before writing any code.
+2. Commit work to that branch.
+3. Open a pull request on Gitea targeting `main` for review.
+4. Wait for approval before merging.
+
+The PR description should reference the related issue (`Closes #n` or `Refs #n`).
+
 ## Commit Messages
 
 Every commit must reference the relevant Gitea issue.