Backlog Core - Refine custom field writeback reliability and prompt contract hardening

[djm81]

Summary

Implement backlog-core-06 refinements for ADO/GitHub backlog flows:

• deterministic custom field writeback mapping (story points and mapped canonical fields)
• strict tmp import ID contract and parser-aligned prompt scaffolds
• ADO markdown formatting and rich-text normalization hardening
• template steering/process selection and anti-content-loss safeguards
• backlog daily/refine filter hardening incl. --state any and --assignee any
• docs + quickstart updates and module signing/version-bump guardrails

OpenSpec Change

• openspec/changes/backlog-core-06-refine-custom-field-writeback

Acceptance

• writeback uses configured mapped fields deterministically
• tmp import requires stable IDs and fails fast on mismatch
• markdown-capable ADO fields render correctly without duplicate headings
• any explicitly disables state/assignee filtering
• prompts/templates match parser contracts per provider

[djm81]

Implementation synced from OpenSpec change backlog-core-06-refine-custom-field-writeback in PR #311:
#311

Artifacts included:

• openspec/changes/backlog-core-06-refine-custom-field-writeback/TDD_EVIDENCE.md
• openspec/changes/backlog-core-06-refine-custom-field-writeback/CHANGE_VALIDATION.md
• openspec/changes/backlog-core-06-refine-custom-field-writeback/tasks.md (all tasks complete)