Complete batch 012 (golden set diff) and 013 (advisory chat), fix build errors

Sprints completed:
- SPRINT_20260110_012_* (golden set diff layer - 10 sprints)
- SPRINT_20260110_013_* (advisory chat - 4 sprints)

Build fixes applied:
- Fix namespace conflicts with Microsoft.Extensions.Options.Options.Create
- Fix VexDecisionReachabilityIntegrationTests API drift (major rewrite)
- Fix VexSchemaValidationTests FluentAssertions method name
- Fix FixChainGateIntegrationTests ambiguous type references
- Fix AdvisoryAI test files required properties and namespace aliases
- Add stub types for CveMappingController (ICveSymbolMappingService)
- Fix VerdictBuilderService static context issue

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/BinaryIndex/__Libraries/StellaOps.BinaryIndex.GoldenSet/AGENTS.md

@@ -0,0 +1,33 @@
+# GoldenSet Library Charter
+
+## Mission
+Provide foundational data models, storage, and validation for Golden Set definitions - ground-truth facts about vulnerability code-level manifestation.
+
+## Responsibilities
+- **Domain Models**: GoldenSetDefinition, VulnerableTarget, BasicBlockEdge, WitnessInput, GoldenSetMetadata
+- **Validation**: Schema validation, CVE existence check, edge format validation, sink registry lookup
+- **Storage**: PostgreSQL persistence with content-addressed retrieval
+- **Serialization**: YAML round-trip serialization with snake_case convention
+- **Sink Registry**: Lookup service for known sinks mapped to CWE categories
+
+## Key Principles
+1. **Immutability**: All models are immutable records with ImmutableArray collections
+2. **Content-Addressing**: All golden sets have SHA256-based content digests for deduplication
+3. **Determinism**: Serialization and hashing produce deterministic outputs
+4. **Air-Gap Ready**: Validation supports offline mode without external lookups
+5. **Human-Readable**: YAML as primary format for git-friendliness
+
+## Dependencies
+- `BinaryIndex.Contracts` - Shared contracts and DTOs
+- `Npgsql` - PostgreSQL driver
+- `YamlDotNet` - YAML serialization
+- `Microsoft.Extensions.*` - DI, Options, Logging, Caching
+
+## Required Reading
+- `docs/modules/binary-index/golden-set-schema.md`
+- `docs/implplan/SPRINT_20260110_012_001_BINDEX_golden_set_foundation.md`
+
+## Test Strategy
+- Unit tests in `StellaOps.BinaryIndex.GoldenSet.Tests`
+- Integration tests with Testcontainers PostgreSQL
+- Property-based tests for serialization round-trip