Add CONTRIBUTING.md

[cdprice02]

The README states "More detailed contribution guidelines may be added as the project matures." Now that the core architecture is stabilizing, formalize contribution expectations.

Should cover:

• Branching conventions
• PR workflow (squash merge)
• Test expectations (unit + integration via ShellTest harness)
• Lint requirements (cargo clippy -- -D warnings)
• Alignment with ferrish's core principles (safety by default, explicit over implicit, predictable semantics)