feat(plan): the build handoff is first-class — 'go' builds, any moment

Every /plan run ends with the build offer: say go and the same session
stops planning and starts executing the plan (authorization for working-
tree implementation only — commits, pushes, deploys, spend keep their own
gates). The interrupt is live the whole run: 'just build it' mid-plan
states decisions plus inferences in one screen and builds — never a fast-
track into more planning. The inverse is equally live: 'keep asking me
questions' re-opens question rounds past the autonomy default. Startup
mode keeps its one honest pushback for question-impatience, but an
explicit build ask cuts through even there.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: 2 people