Validates workstream name at all entry points — CLI --ws flag,
GSD_WORKSTREAM env var, active-workstream file, and cmdWorkstreamSet —
blocking names that don't match [a-zA-Z0-9_-]+. Also fixes
getActiveWorkstream to use planningRoot() consistently and validates
names read from the active-workstream file before using them in path
joins.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>