fix(lmao): nearest-package.json consumer resolution for trace auto-setup

Replaces the lockfile-walk + LMAO_PACKAGES_DIR escape hatch with
standard resolution semantics: the consumer package is the nearest
package.json above process.cwd() — identical in a checkout, under
Bun's isolated store, and in plain node_modules, with no env var. A
nearest package.json declaring workspaces is a workspace-root run;
its members expand for the cross-package tracer scan (a member's own
package.json is always nearer, so member runs never misclassify).

autoSetupBunTestTracing now takes the resolved target ({packageRoot}
or {workspaceRoot, packageRoots}) instead of a packagesDir,
importing the package's src/test-suite-tracer.ts directly and writing
.trace-results.db at the package root (workspace root for cross-package
runs) rather than cwd-relative. Failure to resolve stays a loud stderr
notice — disabled tracing is never a silent no-op.