chore: align repo scaffolding with the package standard

[TheGreatAxios]

Closes CL-9388

Summary

• LICENSE: the Interchange LGPL-2.1 text, byte-identical across @corbits repos (was the FSF text); add CLA.md and the cla.yml CLA Assistant workflow
• package.json: drop main/types (exports covers Node 24 and Bun); add keywords; homepage points at #readme; scripts are the standard set: build clears dist first, test runs bun test src --pass-with-no-tests and test:e2e runs e2e, and a new check runs typecheck, lint, format check and unit tests. Key order is oxfmt's
• tsconfig.json and tsconfig.build.json follow the standard templates: drops the intx-src custom condition, composite, allowJs and the declaration settings from the typecheck config; the build config extends it. No source changes were needed
• .gitignore: standard list (adds *.tgz, coverage/, .env*, .DS_Store)
• ci.yml: bun run check, bun run test:e2e, then a pack-and-import smoke under Node 24 that checks embedTexts, probeEmbedDims and EmbeddingRequestError; drops the single-entry matrix and bun-version: latest
• AGENTS.md: purpose, layout, rules and local development, with the invariants that were in ARCHITECTURE.md
• Delete PRODUCT.md, ARCHITECTURE.md and IMPLEMENTATION.md

Stacked on #16.

• CONTRIBUTING gets a Releasing section: releases are manual, as in Interchange (npm version, push tags, gh release create --generate-notes, npm publish).

Commits

1. chore: use the Interchange LICENSE text and add the CLA
2. build: align package.json and tsconfig with the package standard
3. ci: run check and a Node pack smoke
4. docs: trim CONTRIBUTING, add AGENTS.md, drop planning docs

Checks

bun run build, bun run check and bun run test:e2e pass; the Node pack smoke passes locally.