Node: Standardize dual-format packaging to match established codebase patterns

This Pull Request fixes the packaging for CJS consumers of the Node package.
Previously, the CommonJS entry failed due to a Rollup bundling configuration issue in @herb-tools/node. The package used separate .cts/.mts source files, but Rollup wasn't properly bundling local dependencies for the CJS build, leaving runtime requires that failed.
This change adopts the established pattern: a single src/index.ts source file that Rollup transforms into both ESM and CJS outputs. This matches the proven strategy used by @herb-tools/core, @herb-tools/formatter, @herb-tools/highlighter, and @herb-tools/linter.

This change adopts the established pattern: a single src/index.ts source file that Rollup transforms into both ESM and CJS outputs. This matches the strategy used by @herb-tools/core, @herb-tools/formatter, @herb-tools/highlighter, and @herb-tools/linter.

No public API changes.

Expected vs Actual

• Expected
  • Both ESM and CJS consumers can import/require @herb-tools/node, call await Herb.load(), and use the API.
• Actual (before)
  • ESM import works.
  • CJS import fails at runtime when using the previously split sources.

Reproduction (before)

Using the built artifacts, with the previous split .mts/.cts sources:

1. ESM import (works)

// test-esm.mjs
import { Herb } from "@herb-tools/node"

await Herb.load()
console.log("ESM works:", Herb.version)

Run:

node test-esm.mjs
# ✓ Loads and runs successfully

2. CJS import (fails)

// test-cjs.cjs
const { Herb } = require("@herb-tools/node")

Run:

node test-cjs.cjs
# ✗ Error: Cannot find module './node-backend' (from dist/herb-node.cjs)

Even with the path changed, there'd still be the error:

TypeError: this.backendPromise is not a function
    at HerbBackendNode.load (.../node_modules/@herb-tools/core/src/herb-backend.ts:...)

Diagnosis (before)

CJS build problem:

• src/index-cjs.cts contained require("./node-backend.js")
• Rollup processed this as a runtime dependency, not a bundled dependency
• The built dist/herb-node.cjs expected ./node-backend.js to exist at runtime, but it wasn't emitted
• Additionally, the CJS version passed new Promise(...) (Promise instance) instead of () => new Promise(...) (Promise factory)

ESM build worked because:

• Rollup properly resolved and bundled the local node-backend.ts dependency
• It used the correct Promise factory pattern: () => new Promise(...)

Fix

Adopt the standard, project-wide packaging pattern:

1. Use a single source file: src/index.ts for both formats.
2. Update Rollup to emit both ESM and CJS outputs from that single source.
3. Remove the separate .cts/.mts sources to avoid divergence and bundling issues.
4. Update package.json type entries to the standard ./dist/types/index.d.ts.

This mirrors the approach in other packages and eliminates the non-working CJS path.

Files Changed

• src/index-esm.mts → src/index.ts (renamed, now single source)
• src/index-cjs.cts (deleted, was non-working code path)
• rollup.config.mjs (updated to use standard pattern)
• package.json (updated type declarations)