docs: add Sourcey reference and llms.txt

[IdalunaLabs]

Sourcey docs adoption

Adds a small Sourcey-oriented docs page and docs/llms.txt for scafld.

Why this is useful:

• gives agent readers a compact map of the governed scafld lifecycle;
• records the pinned source commit used for the reference;
• keeps the docs inside the project tree rather than a disposable external preview;
• supports Frantic/runx-style receipt workflows where reviewers need stable public references.

Generation/validation notes:

• Sourcey version observed: 3.6.5
• runx version observed: runx-cli 0.6.14
• pinned source commit used for reference: b705202