feat: add JSON Schema for export and status command output

[jimisola]

Summary

Closes #315

• Add status_output.schema.json — JSON Schema (draft 2020-12) defining the status --format json output contract
• Add export_output.schema.json — JSON Schema defining the export command output contract
• Update codegen and codegen-check scripts in pyproject.toml to exclude *_output.schema.json files from Pydantic model generation
• Add 20 schema validation tests (meta-schema validity, positive/negative document validation)

Design changes from current output format:

• Dropped nr_of_ prefix everywhere
• Nested totals into logical groups (requirements, svcs, tests, automated_tests, manual_tests)
• Renamed implementation → implementation_type, accessible_nodes_dict → import_graph
• Renamed annotations_impls/annotations_tests → annotations.implementations/annotations.tests
• Removed internal indexes (svcs_from_req, mvrs_from_svc, etc.) — relationships derivable from object fields
• Added metadata section with initial_urn and filtered
• Split revision into structured { major, minor, patch } object

This PR creates the schema specification only. No changes to command code or serialization — implementation deferred to Phase 2 (#313).

Test plan

• Both schemas pass JSON Schema draft 2020-12 meta-validation
• Minimal and full example documents validate against schemas
• Invalid documents correctly rejected (wrong enums, missing required fields, extra fields, negative counts)
• codegen-check passes (output schemas excluded from Pydantic codegen)
• Full unit test suite passes (223 tests)