Add CONTRIBUTING.md documenting build workflow, PR process, and branch strategy

[johnmccrae]

Context

There is no CONTRIBUTING.md in the repo. New contributors (and AI agents) must infer the build workflow, PR format, branch strategy, and test commands from the code itself. This was a recurring pain point through ex1-ex11.

Acceptance Criteria

• CONTRIBUTING.md created at repo root covering:
  • Prerequisites (PowerShell 7+, required modules)
  • First-time setup: .\Build\build.ps1 -ResolveDependency
  • Branch naming convention: walk-ex##, run-ex##, etc.
  • Build commands: Stage, Analyze, Test tasks and their order
  • PR process: use .github/pull_request_template.md, sign commits with -s
  • How to update contracts (PlasterTemplate manifests) intentionally
• Cross-referenced from README.md

Relevant Code Paths

• Build/build.psake.ps1 — task definitions to document
• .github/pull_request_template.md — PR process (ex11)
• README.md — add link to CONTRIBUTING.md

Dependency

Blocked by #28 (PR template must exist before CONTRIBUTING.md references it).