chore: sanction reaching-defs as an additive ddg prov token (schema decisions + neo4j lockstep)

Strengthens the code comment at src/schema/v2/dataflow.ts to record that
"reaching-defs" is a deliberate, sanctioned additive DDG prov token — a
documented deviation from the canonical "ssa" tag — accepted across
JSON, Neo4j (open string[] property, no enum), and the SDK. Full
rationale recorded in .claude/SCHEMA_DECISIONS.md (gitignored, local-
only). No emitter behavior change: the token stays "reaching-defs" and
test/schema-v2.test.ts's assertion is untouched.

Author: rahlk