fix(test): clean before build:test so orphan dist cannot mask deleted sources

[Astro-Han]

Summary

build:test was an incremental tsc build that never removed the compiled output of deleted sources. When a source file and its test were removed (e.g. composer-skill-picker in #1915), the orphan dist artifacts survived, and node --test "dist/**/*.test.js" picked up the stale test, which then crashed against the deleted component — surfacing as mysterious undefined.label failures. Clean runs (npm test) and CI (fresh checkout) were unaffected; only the local build:test/test:fast path was.

This PR makes build:test the single owner of dist freshness: it runs npm run clean first, so dist is always derived from current sources and orphan artifacts cannot exist. Full rebuild costs ~6.5s locally (measured), so there is no practical incremental-vs-clean tradeoff. test/test:full no longer repeat the clean (build:test owns it).

Closes #1941 (the issue is a stale-dist false positive; the clean-build gate removes the mechanism that produced it).

Verification

• Planted an orphan file in packages/ui/dist, ran npm run build:test (6.53s real, exit 0), confirmed the orphan was removed by the leading clean.
• Clean builds now pass the affected suites: @maka/ui 247/247, @maka/cli 467/467, @maka/desktop 1400/1400; the stale runtime/dist/__tests__/agent-team-tools.test.js orphan is gone.
• npm run test:scripts 23/23, npm run format:check clean.
• Independently reviewed (gpt-5.6-sol): PASS, no P0–P3; confirmed first-principles fit (test-consumed compile tree must derive from current sources, fail closed) and that orphan detection in check-stale-dist.mjs would be inferior (mtime-only, not wired into the test path).