[Follow-up] Harden backlog refine prompt scaffold and mixed-format parsing

[djm81]

Summary

Follow-up bugfixes for backlog-core-03-refine-writeback-field-splitting based on real-world usage in v0.30.3.

Problems Observed

1. Copilot refinement output format can drift and cause parser misclassification when metadata appears before description.
2. Mixed heading/inline-label output (for example inline **Notes**: inside ## Description) can drop early description narrative.
3. Prompt contract is not explicit enough about expected scaffold ordering.
4. Unknown metadata fields can be emitted as placeholders ((unspecified), no info provided, provide area path) and then interpreted as actual content.

Proposed Fix

• Harden parser boundaries for mixed heading + label formats.
• Preserve ## Description narrative content in mixed-format outputs.
• Strengthen Copilot prompt with explicit ordered scaffold.
• Explicitly require omission of unknown metadata fields (no placeholder text).
• Align --export-to-tmp Copilot instructions with interactive prompt contract.

OpenSpec

• Change ID: backlog-core-03-refine-writeback-field-splitting
• Source change proposal: openspec/changes/backlog-core-03-refine-writeback-field-splitting/proposal.md