chore: establish develop branch, update CI and contributing docs

Add `develop` to CI push trigger so post-merge CI runs on the
integration branch. Add branching note to CONTRIBUTING.md.

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

Author: prestwich

.github/workflows/rust.yml

@@ -3,7 +3,7 @@ name: CI # Continuous Integration
 on:
   workflow_dispatch:
   push:
-    branches: [main]
+    branches: [main, develop]
   pull_request:
 
 # simplest example of using the rust-base action

CONTRIBUTING.md

@@ -22,6 +22,11 @@ We maintain code quality and style standards, and require commit signing. This
 option allows us to make small changes to your PR to bring it in line with
 these standards. It helps us get your PR in faster, and with less work from you.
 
+### Branching
+
+PRs should target the `develop` branch. The `main` branch is reserved for
+stable releases and receives merges from `develop` at release boundaries.
+
 ## Development Basics
 
 Before submitting a PR we recommend you run the following commands to ensure