Preserve all four sub-agent report headings with None. for empty sections

[TheGreatAxios]

Summary

• The leaf report contract requires all four envelope headings in order, with None. under headings that have nothing to report
• The report formatter always renders Summary, Findings, Blockers, and Paths, substituting None. for empty Findings, Blockers, and Paths bodies

Verification

• bun test src/subagent/index.test.ts src/agent/prompts.test.ts passes (80 pass, 0 fail)
• bun run check passes (6981 pass, 0 fail)

Fixes CL-7494

[linear-code]

CL-7494