Define UG_ENABLE_MODEL_DISCOVERY policy

[andy-xu-db]

🥞 Stacked PR

Use this link to review incremental changes.

• stack/codex/model-discovery-flag [Files changed] ← this PR
  • stack/codex/model-discovery-policy [Files changed]
    • stack/codex/codex-scoped-model-discovery [Files changed]

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Summary

• Define the shared UG_ENABLE_MODEL_DISCOVERY environment-variable contract for provider- and Unity Catalog location-scoped agent catalogs.
• Centralize precedence as force → transient launch override → environment, with discovery enabled by default and disabled only by exact UG_ENABLE_MODEL_DISCOVERY=0.
• Keep ordinary system.ai discovery outside this policy.

Scope boundary

• This foundation is behavior-neutral: it adds only the shared constant, policy helper, and direct truth-table tests.
• It adds no managed-config field, CLI behavior, agent state, cache access, or launch behavior. Control Claude scoped model discovery with UG_ENABLE_MODEL_DISCOVERY #672 and Control Codex scoped model discovery with UG_ENABLE_MODEL_DISCOVERY #689 are the Claude and Codex consumers.
• Refresh Claude model cache before scoped launches #622 is not an ancestor and is not part of this stack.

Verification

• Head: 74fa448bfdec48173e9844fcb3dbc216e42facb1.
• Policy truth table: 8 passed.
• Discovery stack top: 2,415 passed and 42 skipped locally, excluding the native-binary capture test whose two environment-dependent failures reproduce on main.
• Ruff and git diff --check passed.