Draft release notes and an LLM go/no-go readiness advisory with four Agent SDK subagents

Notes & tips

• Depend on the documented public surface. The supported API is ReleasePreparationWorkflow and its async execute, and the WorkflowResult it returns. Names with a leading underscore — _run_agent_prep, _SUBAGENT_NAMES, _DEPTH_MAX_TURNS — are internal and may change.
• Draft, then gate. Use release-notes to write the changelog and read readiness, then run release-prep (release-gate) to gate the actual ship on measured numbers.
• Use depth to trade coverage against cost. A quick pass is far cheaper than a deep one; ATTUNE_MAX_BUDGET_USD=0 lifts the cap when a pre-release run must finish.
• Take the changelog from final_output. Release-notes returns the drafted notes in the result; review and place them.