docs: add instructions for building with local protos

[ivanauth]

Description

Adds comprehensive instructions to CONTRIBUTING.md for testing proto changes locally before publishing to the Buf Registry.

Changes

• Added new section "Building with locally modified protos" with 5-step workflow
• Fixed inaccurate reference from "shebang" to "constants" in documentation
• Updated markdown link to point to correct line in magefiles/gen.go (#L26 instead of #L27)
• Added note explaining version metadata behavior when using local protos

Context

Closes #137

Contributors requested clear documentation on how to build locally modified protos. The existing documentation mentioned "mage commands" but didn't explain the local development workflow.

Testing

Documentation-only change. Only CONTRIBUTING.md was modified. No code or tests affected.