feat: target-ontology-adapter §1 Foundation

[deanban]

Foundation scaffolding for the target-ontology-adapter capability: package layout, test directories, and the one new direct dependency (pyyaml).

Tracking: #97. Source: openspec/changes/target-ontology-adapter/tasks.md §1.

Tasks

• 1.1 Create src/sema/targets/ package with protocol surface re-exports
• 1.2 Create src/sema/targets/adapters/ subpackage
• 1.3 Create src/sema/models/target/ package with DTO surface
• 1.4 Add pyyaml to [project.dependencies]; uv sync
• 1.5 Add tests/unit/targets/ and tests/unit/models/target/ with fixtures
• 1.6 Add tests/integration/targets/ for Neo4j round-trip tests
• 1.7 Confirm unit/integration markers in pyproject.toml