Downgrade read-only maintenance workflows from Sonnet to Haiku

[Copilot]

Several read-only maintenance workflows were still pinned to frontier models despite only doing review/summarization work. This changeset moves the highest-frequency offenders to claude-haiku-4.5 to reduce per-run cost without changing workflow behavior.

• PR Description Updater

  • Added an explicit top-level Copilot model pin to claude-haiku-4.5
  • Downgraded the pr-description-synthesizer sub-agent from the generic large tier to claude-haiku-4.5
  • Left the chunk-analysis structure intact; only model selection changed

• Read-only reviewer workflow sweep

  • Downgraded these read-only maintenance reviewers from claude-sonnet-4.6 to claude-haiku-4.5:
    • impeccable-skills-reviewer
    • mattpocock-skills-reviewer

• Compiled workflow outputs

  • Recompiled the affected .lock.yml files so generated workflow metadata and runtime model wiring match the markdown sources

• Contract coverage

  • Added a focused workflow contract test to guard the cheaper model selections and prevent regressions back to Sonnet / large

engine:
  id: copilot
  model: claude-haiku-4.5

## agent: `pr-description-synthesizer`
---
model: claude-haiku-4.5
---