feat: add legacy CJS decorator entrypoint

[sjungwon03]

Summary

• Add @theorvane/type-chain/legacy for TypeScript experimentalDecorators CommonJS consumers.
• Provide legacy Agent, Tool, and Policy metadata registration without changing the root Stage 3 API.
• Bundle the legacy entrypoint as ESM/CJS and defer optional LangChain imports to async runtime adapters.
• Add legacy fixture, packed CommonJS consumer verification, API documentation, and generated-fixture ignore rules.

Linked issue

Closes #94

Verification

• npm run verify (Biome, typecheck, 52 tests, build, package artifact check)
• npm run verify:consumer (packed standard and legacy CJS consumers)
• git diff --check

Compatibility

./legacy supports public instance methods with string names only. toLangChainTools and buildAgent are async because optional LangChain dependencies load dynamically; standard and legacy decorators must not be mixed in one TypeScript compilation unit.

[sjungwon03-ai]

Blocking on exact-head CI: npm 12 release readiness fails in scripts/verify-legacy-consumer.mjs because npm pack --json is destructured as an array while npm 12 returns a non-iterable object (existing scripts/pack-json.mjs already handles both shapes).

[sjungwon03]

Addressed the npm 12 packed-consumer failure in ba843d9: verify-legacy-consumer.mjs now reuses getPackedTarballFilename() for both npm 11 array and npm 12 package-keyed JSON. Added a release-workflow contract assertion so the legacy script cannot bypass that normalization. Local npm run verify and npm run verify:consumer pass.

[sjungwon03-ai]

Exact-head re-review of ba843d9: the legacy packed-consumer script now delegates npm pack JSON handling to getPackedTarballFilename(), preserving npm 11 array output and supporting npm 12 package-keyed output. Contract coverage and packed CommonJS consumer verification passed under npm 11.5.1 and 12.0.1; current CI is green. No blocking findings.

[sjungwon03-ai]

The prior npm 12 packed-consumer blocker is resolved through shared tarball JSON normalization, and the exact-head CI workflow completed both required checks successfully. The published legacy ESM/CJS contract and consumer verification are consistent with the documented compatibility boundary.