Core walkthroughs author single-condition steps with `signal:`, `check:` or
`route:` directly on the step -- 24 sites in frontend/src/data/walkthroughs.js
do it -- and the frontend registry's completionFor() has always desugared
them. The backend validator did not: `_STEP_FIELDS` rejected all three, so an
extension that copied the pattern passed `new-extension --validate` and was
then refused at install with an "unsupported fields" error.
Teach the backend the same shorthand rather than take it away from core.
`_completion_for()` mirrors completionFor() exactly (an explicit `completion`
block still wins) and returns which spelling supplied the value, so problems
are reported against the key the author wrote -- `steps[0].signal`, not
`steps[0].completion.signal`. The shorthand is a spelling, not an escape
hatch: the same token regexes and length bounds apply either way.
The published schema gains the three properties so the contract, the
validator and the frontend now agree.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Extensions can now contribute walkthroughs via contributions.walkthroughs
in plugin.json. The backend validates definitions against a shared
contract (route, signal, manual, and check completion types), transports
them through the dynamic contribution envelope, and persists custom
walkthroughs from the Studio. Includes the JSON Schema, SDK version gate,
and extension scaffold support.
