Add model lifecycle guardrails: centralized registry, preflight validation, and stable default chain

[Copilot]

Model deprecations have been repeatedly breaking critical workflows due to hard/implicit pins to retired or unavailable models. This PR introduces a lifecycle control layer so workflows fail fast with actionable errors and default resolution paths avoid unstable model versions.

• Model lifecycle source of truth

  • Added .github/workflows/shared/model-versions.json as a central registry of:
    • active defaults per engine/provider
    • retired/unavailable models
    • successor mappings for migrations

• Fail-fast preflight checks in impacted workflows

  • Added a Pre-flight model lifecycle validation step to:
    • .github/workflows/pr-sous-chef.md
    • .github/workflows/pr-code-quality-reviewer.md
    • .github/workflows/daily-model-resolution.md
  • Each workflow now validates pinned models against the registry before agent execution and emits targeted errors with successor hints.

• Stable default chain migration

  • Updated builtin alias defaults to remove deprecated gpt-5.5 from the agent fallback chain in:
    • pkg/workflow/data/model_aliases.json
  • Synced docs/tests for the alias chain update:
    • docs/src/content/docs/reference/model-tables.md
    • pkg/workflow/model_aliases_test.go

• Inventory workflow upgraded for proactive deprecation detection

  • Extended .github/workflows/daily-model-inventory.md to include lifecycle-retirement scanning against:
    • workflow model pins
    • sub-agent model pins
    • default alias chains

• Model pinning guidance

  • Added concise pinning/lifecycle guidance in SKILL.md to bias critical workflows toward stable, explicit model pins.

# Example preflight gate added to affected workflows
- name: Pre-flight model lifecycle validation
  shell: bash
  run: |
    REG=".github/workflows/shared/model-versions.json"
    status="$(jq -r --arg model "copilot/gpt-5.4" '.models[$model].status // "unknown"' "$REG")"
    successor="$(jq -r --arg model "copilot/gpt-5.4" '.models[$model].successor // empty' "$REG")"
    case "$status" in
      active) echo "validated model lifecycle entry" ;;
      retired|unavailable) echo "::error::model unavailable; successor: $successor"; exit 1 ;;
      *) echo "::error::model missing from registry"; exit 1 ;;
    esac

[github-actions]

Hey @Copilot 👋 — great work on this model lifecycle guardrails PR! Introducing a centralized model-versions.json registry with fail-fast preflight checks and a cleaned-up default alias chain is exactly the kind of resilience improvement that makes agentic workflows more reliable.

The PR is well-structured, has a clear description, updates tests and golden files throughout, and keeps all changes tightly scoped to the model lifecycle theme. It looks ready for review. 🎉

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