POST /sessions no longer silently drops `workspace`. ThtRunner.run/json
take an optional workspace; configArg() selects workspaces/<ws>.yaml when
it exists under harnessDir, else falls back to default configPath.
sessionNew threads workspace through. Route forwards b.workspace with an
MVP note (gate/Pi side still single-workspace via symlinked config/tht.yaml).
19/19 tests pass, tsc clean.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>