Improve validator error aggregation and fail-fast behavior

[joy7758]

This PR improves validator error reporting while keeping the existing default validation path compatible.

Summary

• Adds explicit fail_fast control to build_validation_report(...) and validate_profile_file(...).
• Keeps default validation behavior fail-fast by stage to preserve existing fixture and CLI expectations.
• Adds top-level flattened issues, primary_error_code, and issue_summary fields to validation reports.
• Marks skipped stages explicitly with skipped and skip_reason.
• Supports fail_fast=False to aggregate structurally safe later-stage errors after schema validation passes.

Scope

This PR only modifies the operation accountability profile validator and its tests.

It does not:

• modify public schema
• modify application runtime behavior
• modify CLI command wiring
• restore any stash
• touch paper, AGT, Review Pack, poster, submission, or roadmap materials

Validation

• git diff --check: passed
• ruff check: passed
• ruff format --check: passed
• pytest: 81 passed, 1 skipped, 15 warnings