chore: track .claude/SCHEMA_DECISIONS.md (un-ignore the schema-decisions doc)

[rahlk]

.claude/SCHEMA_DECISIONS.md is the designated home for schema/parity decisions (referenced from CLAUDE.md), but it is caught by the global gitignore (.claude/), so those decisions are not version-controlled or discoverable by other devs / the SDK — e.g. the sanctioned reaching-defs token recorded for #32 lives only locally.

Un-ignore it: add !.claude/SCHEMA_DECISIONS.md to the repo .gitignore, git add -f, and verify git ls-files .claude/SCHEMA_DECISIONS.md.