Align development prerequisite guidance

[Copilot]

The repository and docs-site development guides gave conflicting guidance on whether Go is required. This created uncertainty during setup and allowed the two onboarding paths to drift.

• Prerequisites: Mark Go optional, limited to non-release compiler builds; document curl/wget consistently.
• Source of truth: Designate DEVELOPING.md as authoritative and cross-link both development entry points.
• Verification: Clarify that go version applies only when building from source.

- Go — optional; only needed for non-release gh-aw compiler builds
- curl or wget — used to download the official gh-aw release compiler binary

• Fixes [information-architecture] Conflicting Go prerequisite guidance across development entry points #1897