chore: align repo scaffolding with the package standard

[TheGreatAxios]

Closes CL-9398

Summary

• LICENSE replaced with Interchange's LGPL-2.1 text (was the FSF text). CLA.md and the CLA Assistant workflow (cla.yml, signatures on a cla-signatures branch) added; CONTRIBUTING links the CLA.
• package.json: dropped main/types (exports covers Node and Bun), added engines bun >=1.2.0 alongside node >=24, test runs bun test src --pass-with-no-tests, test:e2e runs bun test e2e, and a check script; key order is oxfmt's sortPackageJson order.
• tsconfig.json and tsconfig.build.json use the shared templates (adds noUncheckedIndexedAccess, exactOptionalPropertyTypes, noImplicitOverride, moduleDetection). Fixes for what they surfaced:
  • src/migrations.ts: ssl: config.ssl ?? false instead of a possibly undefined ssl (the driver's default)
  • src/frame.test.ts: chain[2]! for an index that always exists
  • e2e/sse-stream.test.ts: sets heartbeatIntervalMs on the deps only when the test passes one
  • e2e/sse.test.ts: readUntil takes Pick<ReadableStreamDefaultReader<Uint8Array>, "read">, and e2e/helpers.ts lets as/jsonAs infer their Headers type (the Node and Bun global types disagree under the new flags)
• .gitignore replaced with the shared template.
• test.yml replaced by the standard ci.yml: checkout v5, Node 24, Postgres 16 on port 5432, bun run check, bun run test:e2e, then packs the tarball and imports createMailboxRoutes and runMailboxMigrations under Node. The e2e/helpers.ts default MAILBOX_TEST_DATABASE_URL moves from port 5433 to 5432 to match.
• CONTRIBUTING cut from 107 to 25 lines (Development, CLA line, Postgres setup, Migrations, Commit messages). The design rules from CONTRIBUTING and ARCHITECTURE.md (ownership boundaries, seams, dual-write, caps, SSE limits) move into a new AGENTS.md, and ARCHITECTURE.md is deleted. README badges removed.
• CONTRIBUTING gets a Releasing section: releases are manual, as in Interchange (npm version, push tags, gh release create --generate-notes, npm publish).
• CHANGELOG.md removed; GitHub Releases track changes.

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 ARCHITECTURE.md

Checks

bun run build, check (46 tests; the migration and write unit tests use Postgres), test:e2e (113 tests, local Postgres) and the Node pack-and-import smoke pass locally. On a cold database the first test:e2e run once hit bun's 5s hook timeout in e2e/isolation-and-purge.test.ts; reruns pass.