feat(config): set GT_ROOT env var for all agent sessions

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>

internal/cmd/doctor.go

@@ -120,6 +120,7 @@ func runDoctor(cmd *cobra.Command, args []string) error {
 	d.Register(doctor.NewRoutesCheck())
 	d.Register(doctor.NewOrphanSessionCheck())
 	d.Register(doctor.NewOrphanProcessCheck())
+	d.Register(doctor.NewGTRootCheck())
 	d.Register(doctor.NewWispGCCheck())
 	d.Register(doctor.NewBranchCheck())
 	d.Register(doctor.NewBeadsSyncOrphanCheck())