Update gh-aw docs for setup prerequisites and triggers

[github-actions]

Summary

• Update docs/developing.md so gh-aw release builds document curl/wget for the official binary and make Go optional for non-release compiler builds.
• Update the gh-agent workflow architecture docs to describe source-template comment/review/discussion triggers while preserving the role of example.yml trigger files.

Validation

• UV_CACHE_DIR=/tmp/gh-aw/agent/uv-cache make docs-build

Closes #1280.

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What is this? | From workflow: Trigger Docs Patrol

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