refactor(codemode): avoid merging root definitions twice

[kitlangton]

Why

JSON Schema rendering constructs a dictionary from root definitions and $defs, then immediately passes it to renderSchema, which copies and merges those same definitions again.

What Changes

Start the root renderer with an empty inherited-definition context. Its existing schema-local merge remains the single owner of root definitions, preserving $defs precedence, nested shadowing, sibling isolation, and compact/pretty output.

Scope

One production line plus focused tests. Effect schema document definitions in toTypeScript are unchanged; no schema, tool API, or interpreter changes.

Verification

cd packages/codemode
bun run test test/signature.test.ts
bun typecheck
cd ../..
bunx prettier --check packages/codemode/src/tool-schema.ts packages/codemode/test/signature.test.ts
git diff --check HEAD^ HEAD

28 signature tests passed before and after the production change. Package typechecking, formatting, and whitespace checks passed. The new cases exercise legacy definitions, $defs precedence, and nested/sibling scope in both rendering modes.