build(packages): emit ESM type marker for publish

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Summary

• Add scripts/write-esm-package-json.js as a shared postbuild step that writes dist/esm/package.json with {"type":"module"}. Without this marker, bundlers (webpack, esbuild, vite, rollup) and older Node versions parse dist/esm/*.js as CommonJS and error on the export syntax emitted by tsc.
• Drop "source": "./src/index.ts" from each package's exports map — src/ is not shipped in the published tarball, so this condition pointed at a path that doesn't exist for consumers.

Why this is the minimum needed to publish

The published shape is dual-CJS+ESM (index.js + esm/index.js). Node and bundlers determine how to parse .js files via the closest package.json's "type" field. Without a marker inside dist/esm/, the ESM build inherits the package root's default ("commonjs") and fails wherever syntax detection isn't available.

Verification

Decisive pack-and-import matrix run against @agentic-kit/anthropic (leaf package, no workspace deps):

Scenario	Marker	Detection	Result
1	❌	✅ Node 22+ default	✅ OK (detection masks the bug)
2	❌	❌ (--no-experimental-detect-module)	❌ FAIL — ERR_REQUIRE_CYCLE_MODULE
3	✅	✅	✅ OK
4	✅	❌	✅ OK — marker IS the fix
5 (CJS require)	✅	✅	✅ OK

Test 2 vs Test 4 isolates the marker as the cause: same Node, same tarball shape minus one file, opposite outcomes.

Test plan

• pnpm clean && pnpm build — all 6 packages emit dist/esm/package.json with {"type":"module"}
• pnpm test — 99 tests pass across 6 packages
• Pack + import ESM from a fresh Node process (with and without --experimental-detect-module)
• Pack + require() from a CJS consumer
• After merge: lerna publish dry-run sanity check before the first real publish

Out of scope

• Lint fix for the pre-existing jest.environment.js eslint-disable directive (lives in a separate branch). CI doesn't gate on lint, so this PR is independently mergeable.
• engines.node floor — keeping the marker means we don't need to commit to a Node version yet.

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