spec hygiene: canonicalize Truth Plane ADR + examples + schema IDs

[mdheller]

This PR cleans up Truth Plane spec authority and reduces ambiguity.

Key changes (no loss of intent):

1. Canonical ADR

• Adds ADR-0009 (TruthSurface/DeltaSurface) and Appendix A as the canonical reuse map.
• Deprecates the colliding ADR-0001 TruthSurface/DeltaSurface files by turning them into stubs pointing to ADR-0009.

2. Canonical examples

• Adds snake_case canonical examples: examples/truth_surface.json and examples/delta_surface.json.
• Removes duplicate hyphenated sample files: truth-surface.sample.json and delta-surface.sample.json.

3. Catalog/docs hygiene

• Updates schemas/README.md to include TruthSurface and DeltaSurface in the quick reference and in the Execution/Provenance family table.
• Updates examples/README.md to include a Truth Plane examples section and add the new files to the file index.
• Adds docs/adr/README.md as an ADR index + numbering rule.

4. Schema ID normalization

• Normalizes control-plane IncidentEvent schema $id into the srcos namespace.

Notes:

• OpenAPI/AsyncAPI truth-plane patch fragments remain additive as-is.
• Enforcement remains in SociOS-Linux/SourceOS.