fix: restore stale test scripts

[Santiagocetran]

What's going on

pnpm run test:build and pnpm run test:cli don't work right now. They call two scripts (scripts/test-build.sh and scripts/test-cli.sh) that don't exist in the repo anymore.

I dug into the git history and found it: back in November, commit 1d18395e ("upd bundle analyzer metrics script") deleted these two test scripts as a side effect. That commit was really about adding an unrelated bundle-analyzer script, so this looks like an accidental cleanup rather than a deliberate decision. Nobody updated package.json afterward either, so the commands were just left pointing at files that no longer existed. Since then, they've quietly been broken with nobody noticing.

Why bring them back

They're the only thing that actually verifies the CLIs work end to end locally, that xmcp, create-xmcp-app, and init-xmcp build correctly and can each do their basic job (help output, scaffolding a project, initializing one in place). Without them, pnpm run ci (the command you'd normally reach for before opening a PR) silently fails on step one. There's a separate test-split-e2e.sh script for runtime/compiler testing, but it doesn't cover the CLIs at all, so it's not a substitute.

What this PR does

• Restores both scripts, rewritten to match how the project builds and runs today (a few things changed since November, like init-xmcp not existing yet).
• Cleans up a couple of package.json entries that were built around the old broken scripts.
• Removes a now-redundant build step in the manual CI workflow.
• Updates a small section of DEVELOPMENT.md so the docs match what these commands actually do.

Testing

Ran test:build, test:cli, test:ci, and ci:build-only locally end to end, all pass. Worth noting test:cli needs Node 22+ and does real (small) npm installs into a scratch folder, so it takes a bit longer than a typical script.

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