feat: add central capability metadata schema definition

[normenmueller]

Summary

Create the single authoritative schema file for capability metadata at adm/gdl/dev/schemas/capability-meta.schema.yaml using a custom YAML DSL (architecture decision U1).

Changes

• Schema file (adm/gdl/dev/schemas/capability-meta.schema.yaml): Declares 3 required fields (id, version, owner) and 5 optional fields (do_not_use_when, distinguish_from, requires, conflicts, sources) per AC-01 through AC-09.
• Tests (utl/cap/checks/tst/capability-meta-schema.test.mjs): 31 structural tests verifying schema correctness against all applicable ACs.
• Test integration (utl/cap/package.json): Schema tests added to npm test runner.

Architecture Decisions

• U1: Custom YAML DSL format (zero new dependencies, governance-readable)
• U2: All 5 sources entry properties required per entry

Acceptance Criteria

• AC-01: Central schema file at canonical path
• AC-02: Exactly 3 required fields
• AC-03: Exactly 5 optional fields with correct types
• AC-05: Excluded fields absent
• AC-07: owner required with Semantic Authority semantics
• AC-08: do_not_use_when typed as string array
• AC-09: distinguish_from as array of {id, boundary} objects

closes #38