feat(deliver): build each issue of a program in a fresh subagent

[andeeplus]

Long audit and deliver runs reached 150k+ tokens of context, which raised their cost, because each later issue re-read every earlier one.

• Deliver hands each issue's pass (branch, build, gate, PR) to a fresh subagent briefed with the issue, its draft and the project instructions. The run keeps only the issue's output block.
• Audit's Execute section passes each subagent its draft path and audit entries; between issues the run holds only manifest.json and the output blocks.

Not verified: a subagent running final-review's own subagents. If it cannot, the gate runs inline inside the subagent.