Previously GT_ROOT was documented as a formula search path but never
actually set, making $GT_ROOT/.beads/formulas/ unreachable for agents.
Now BuildStartupCommand automatically sets GT_ROOT to the town root,
enabling all agents (witness, refinery, polecat, crew, etc.) to find
town-level formulas without relying on cwd-relative paths.
Also adds a doctor check (gt-root-env) that warns when existing sessions
are missing GT_ROOT, with instructions to restart sessions.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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