[P0] Surface skipped models and parser failures as structured diagnostics

[jackyclever]

CLI scope

Expose extraction coverage and failures as machine-readable diagnostics so consumers can distinguish not found from not indexed.

Split from Aether #4. Aether remains responsible for fail-closed interpretation of these diagnostics.

Current problem

The extraction layer is best-effort in several paths: model/object parse failures, malformed descriptors/XML, unreadable directories, and intentional exclusions can result in omitted objects without a complete structured reconciliation.

Acceptance criteria

• Every discovered model has a terminal state: processed, intentionally excluded, failed, or unreadable.
• Every discovered supported object has a terminal parse state.
• JSON output includes stable diagnostic codes, object/path identity, and sanitized messages.
• Intentional exclusions are counted separately from failures.
• Extraction summary reconciles discovered = processed + intentionally excluded + failed/unreadable.
• Regression tests cover malformed XML, parser failure, access failure, and model-level failure.
• The same diagnostics are available to both CLI and MCP consumers without scraping console logs.