feat: delegate inline-stage artifact drafting to throwaway subagents

[fanhongy]

Summary

Inline ideation/inception stages run their full body in the main conductor
context, so heavy artifact-drafting reasoning (large consumes, full produces
drafts) accumulates in the main session and forces the user to compact
repeatedly — and the forwarding loop won't yield for a compaction without a
manual interrupt.

This adds a protocol-driven drafting hand-off (ralph-loop style): the
conductor stays thin — it collects answers and runs the approval gate inline —
but delegates only the artifact-drafting sub-step to a single throwaway
Task(directive.lead_agent) whose context is discarded after it returns the §11
summary. The heavy drafting reasoning never lands in the main transcript.

Stages stay mode: inline — no engine, schema, or directive-type change.
This is deliberately distinct from whole-stage mode: subagent, which an
interactive stage cannot use (a subagent cannot drive the question UI or the
approval gate).

Approach

Chosen over wiring the unused dispatch-subagent directive because the split we
need is intra-stage: it fits inside the one existing run-stage directive, so
it needs no engine/schema/directive churn and keeps the blast radius to prose +
docs + one test. The draft step slots into the existing atomic stage ritual
(questions → artifact → reviewer → completion → learnings → gate) — it
specifies how the artifact step runs, not a new step.

Hook-safety

• The conductor never spawns the draft Task while any [Answer]: tag is still
  blank (the §3 Step 4 completeness check gates it).
• The subagent treats any *-questions.md in the stage dir as read-only
  (the Stop hook matches the -questions.md suffix, not just the canonical
  name), so it can never introduce/clear a blank tag that aidlc-stop.ts's
  Tier-2 human-wait carve-out would misread.

Changes

• core/aidlc-common/protocols/stage-protocol.md — new "Drafting delegation for inline stages" section (three-part execution, ordering, hook-safety, fallback)
• harness/claude/skills/aidlc/SKILL.md — draft sub-step in the gate: true arm
• core/aidlc-common/conductor.md — persona craft for the delegation
• docs/reference/03,04,15 — document the hand-off, distinct from mode: subagent
• tests/integration/t154-drafting-delegation-contract.test.ts — locks the contract across protocol + SKILL
• dist/ — regenerated for all four harnesses (drift-clean)

Scope notes

• This lands the shared mechanism; it applies by shape to every gated
  artifact-generating inline stage. Verify on the heaviest stages
  (requirements-analysis, application-design) before relying on it broadly.
• Versioning intentionally left to maintainers — this PR does not touch
  aidlc-version.ts, CHANGELOG.md, or the README badge. Happy to add the
  bump/changelog entry if you'd prefer it in-PR.

Testing

• bun scripts/package.ts --check — drift-clean across all four harnesses
• t154 (new), t32, t37, t38, t68, t119 — all pass
• Full default tier: 181/183 files pass; the 2 failures (t140, t19) are a
  pre-existing missing @anthropic-ai/claude-agent-sdk dependency in the local
  environment, unrelated to this change.

Refs #421

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