docs: improve fork contribution workflow in CONTRIBUTING.md #218

Problem

CONTRIBUTING.md currently assumes contributors have direct push access to the repo. Contributors working from forks encountered several gaps that led to workflow mistakes:

1. No fork setup instructions — Getting Started shows git clone https://github.com/bradygaster/squad.git but does not explain how to fork first, clone the fork, and add upstream as a remote.

2. gh pr create has no --base dev flag — The Pull Request Process section says gh pr create with no base specified. This defaults to main, which is wrong. External contributors must target dev explicitly: gh pr create --base dev --repo bradygaster/squad --head username:branch.

3. Changeset not in the PR checklist — The Changesets section documents how to add a changeset, but the PR Process numbered list doesn't include it as a required step. Easy to skip.

4. Branch Strategy misleadingly describes bradygaster/dev — Currently says "Brady's integration branch (team usage)" which implies it's internal-only. It should clearly say it's the PR target for all contributions.

5. No guidance on rebasing before opening a PR — Nothing explains that the fork branch must be rebased on upstream/dev to avoid conflicts before creating the PR.

Proposed Changes

• Add a "Contributing from a Fork" section covering the full fork workflow
• Update Pull Request Process to specify --base dev and add changeset as a numbered step
• Fix Branch Strategy description of bradygaster/dev
• Add a "Keeping Your Fork in Sync" subsection with rebase instructions

Context

Discovered during PR #217.