decomposer(plan): intent into milestones, scoped issues, acceptance criteria, dependency order

[JSONbored]

Problem

The current expansion produces gate-ready issues. A project needs plan-level structure:
what ships in what order, what blocks what, and which issue satisfies which criterion.

Scope

• Produce from an intent record: milestones with a coherent shipping order; epics where a
  goal needs several issues; scoped issues with the acceptance detail the gate already
  expects; the criteria mapping (apr(completion): criteria → issue coverage mapping and gap detection #10082); and explicit dependency edges
• Size issues for single-contributor completion — the observed failure mode is issues too
  large to finish cleanly, which shows up as revision churn in the quality score
• Sequence so early milestones produce something demonstrable, not just foundations
• Human approval before posting stays in v1 (it is the current CLI's behaviour and it is
  correct while quality is unproven)

Acceptance

An intent record produces a complete plan — ordered milestones, sized issues, criteria
mapping, dependencies — that a contributor can start on without asking a question.

Depends on

#10081 (criteria record), #10082 (coverage mapping).

[linear-code]

JSO-7090