fix(packages): publish Bun entrypoints from dist

[glockyco]

Fixes the npm package export regression introduced in the 4.0.0/3.0.1 release.

Problem

The public package manifests declared:

"exports": {
  ".": {
    "bun": "./src/index.ts",
    "import": "./dist/index.js"
  }
}

but the published npm packages only include dist/ (plus schemas for protocol). Bun prefers the bun condition, so Bun consumers could not import @hotrepl/sdk from npm at all:

Cannot find module '@hotrepl/sdk'

Fix

• Point public Bun entrypoints at ./dist/index.js for protocol, sdk, cli, and mcp.
• Add a package-export regression test that asserts public export targets are included in package files and Bun entrypoints use built JS.
• Run package tests after build so workspace tests exercise the same public entrypoint shape consumers receive.
• Update SDK tests that compare SDK error classes to import @hotrepl/sdk, avoiding duplicate source-vs-dist constructors after build-before-test.

Release

Patch changeset:

• @hotrepl/protocol 4.0.1
• @hotrepl/sdk 4.0.1
• @hotrepl/cli 3.0.2
• @hotrepl/mcp 3.0.2

Verified locally:

• bun test packages/sdk/test/package-exports.test.ts fails red before the metadata fix and passes after.
• bun run test && bun run typecheck && dprint check && bun changeset status --verbose.
• npm pack tarballs installed into a clean app with npm, then imported under Bun: sdk/protocol/cli/mcp all load.