Handrix developer docs

Code-side documentation for Handrix: pulling a project locally, running it, syncing custom code back, automating with the API.

Four sections, picked by what you came here for:

• Tutorials — first-run paths from install to commit.
• How-to guides — task recipes (resolve a conflict, add a custom node type).
• Reference — CLI, REST, and webhook surfaces.
• Explanation — architecture and design rationale relevant to integrators.