Preserve nested sub-select branches in canonical IR lowering

[flyon]

Summary

This PR fixes canonical IR fidelity for nested array sub-selects so build() preserves all selected nested paths instead of dropping deeper branches during desugar/lowering.

What changed

• Make DesugaredMultiSelection recursive (DesugaredSelection[]) and preserve all child selection kinds in desugar.
• Recurse through multi_selection in lowering projection seed collection.
• Add regression fixture + tests for nested sub-select preservation (pluralFilteredNestedSubSelect).
• Update affected SPARQL golden for nestedQueries2 to reflect preserved nested branch output.
• Add/update workflow skills for explicit automatic mode and mandatory mode transition gates.
• Add patch changeset: .changeset/nested-subselect-ir-completeness.md.
• Add final implementation report: docs/reports/006-nested-subselect-ir-completeness.md.

Validation

• npm test -- --runInBand src/tests/ir-desugar.test.ts src/tests/ir-select-golden.test.ts src/tests/sparql-select-golden.test.ts
• npm test
• npm run sync:agents

All passed.

Notes

• No new public exports.
• Behavioral change is intentional: canonical IR now includes nested selected paths that were previously dropped.