Add branching patterns operational guide (GitHub Flow, Git Flow, release process)

[Copilot]

ADR-001 documents the decision on branching strategy but provides no actionable how-to guide for developers. This adds a practical reference covering both supported models and their release processes.

New: Guidelines/Branching.md

• Overview — side-by-side comparison table of GitHub Flow vs Git Flow
• GitHub Flow — default for all new repos; covers feature dev, release (tag from main), and hotfix workflows
• Git Flow — for operationally critical/large/multi-version repos; covers feature dev, release branch lifecycle (develop → release/x.y.z → main → tag → back-merge), and hotfix workflows
• Choosing a Model — GateKeeper decision criteria (distilled from ADR-001)
• Migration checklist — steps to upgrade a repo from GitHub Flow to Git Flow
• External Contractors — links to the Integration Delivery Workflow covering upstream/upstream/<vendor> branches, x.y.z-upstream.N prerelease tagging, and formal acceptance via PR

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[tlmquintino]

@copilot please rebase this PR and move the file to be part of the Guidelines directory with the name Branching.md

[Copilot]

@copilot please rebase this PR and move the file to be part of the Guidelines directory with the name Branching.md

Done in 85a8e63. The file has been moved to Guidelines/Branching.md (alongside the existing pr_guidelines.md), the Branching/ directory has been removed, and README.md is restored to its original state.