Harden structured Command operations across provider capabilities #44

Goal

Preserve Lucid's checked Ops → BlockPatch safety model across providers with different structured-output behavior.

Depends on

• Define a provider-neutral AI client and capability contract #35 and the relevant provider adapter issues.

Scope

• Classify structured output as native, prompted-and-validated fallback, or unavailable.
• Validate every response with the existing schema and compiler.
• Preserve repair limits, stale-target checks, destructive confirmation, single-transaction undo, and unaffected blocks.
• Prevent unsafe Command execution when a provider/model cannot meet the required contract.

Acceptance criteria

• Adapter contract tests cover valid, malformed, partial, and schema-violating responses.
• A capability gap produces an explicit UI state rather than a silent downgrade.
• Existing Command evals run per supported provider configuration.
• This issue extends the current operation system; it does not replace it.