Remove vestigial boxel-ui dist build pipeline

[lukemelia]

What & why

Since 34c836226a "Consume boxel-ui directly", the host and the boxel-ui test-app consume @cardstack/boxel-ui from source — its exports map resolves ./* → ./src/*.gts and ./*.css → ./src/*.css. Nothing compiles or reads boxel-ui's dist/ anymore, so the rollup → dist build and the guards that used to keep dist fresh are dead code.

This is the followup that commit's message anticipated: "keeps the build scripts but makes them into no-ops… A followup PR can eliminate them. Also, the rollup dependencies and config in boxel-ui could get eliminated."

What's removed

boxel-ui's rollup pipeline (packages/boxel-ui/addon):

• rollup.config.mjs
• the prepack and start:js rollup scripts, and the build:js no-op (build/start still run their :types children)
• the rollup, @rollup/plugin-babel, and rollup-plugin-copy devDependencies
• dist from the published files list

The host-side "ensure boxel-ui dist is fresh" guard (packages/host):

• bin/conditional-build.sh
• the ensure-boxel-ui script and its inline execFileSync invocation in scripts/vite-serve.js

Removing the vite-serve.js guard also means pnpm start no longer runs an ember-tsc declaration build on every launch.

Verification

• Fresh-checkout host build — with packages/boxel-ui/addon/dist deleted, vite build --mode=development in packages/host succeeds (✓ built in ~31s), zero embroider-resolver errors. This is the exact scenario from CS-12187.
• pnpm --filter @cardstack/boxel-ui build passes (now build:types only).
• The pnpm-lock.yaml diff is 9 lines, confined to boxel-ui's importer block — only the three rollup deps, no peer re-resolution.

Follow-up (needs workflow OAuth scope)

Two comment blocks in .github/workflows/boxel-cli-publish.yml still describe the host build as reading boxel-ui's dist/ (e.g. "the host's vite build imports … which resolves to the addon's dist/"). Those are now stale — the pnpm --filter @cardstack/boxel-ui build steps stay (they build declarations/), only the comments need correcting. Left out of this PR because the pushing credential lacks workflow scope.

Context: CS-12187 — triage found the reported lodash/dist embroider error is already resolved on main by the source-consumption switch; the error only reproduced against a leftover pre-refactor dist/. This PR removes the machinery that made it look like a build-ordering bug.

🤖 Generated with Claude Code

[github-actions]

Preview deployments

• Host staging preview

• Host production preview

• Boxel UI staging preview

Host Test Results

    1 files      1 suites   3h 3m 41s ⏱️
3 626 tests 3 611 ✅ 15 💤 0 ❌
3 645 runs  3 630 ✅ 15 💤 0 ❌

Results for commit 1d8e2e5.

Realm Server Test Results

    1 files      1 suites   13m 20s ⏱️
1 934 tests 1 934 ✅ 0 💤 0 ❌
2 013 runs  2 013 ✅ 0 💤 0 ❌

Results for commit 1d8e2e5.