fix: remove conflicting required+default in mol-session-gc formula (#919)

vars.mode had both required=true and default="conservative", which
causes formula validation to fail with:

  vars.mode: cannot have both required:true and default

This prevented gt doctor from dispatching cleanup work to dogs.

Remove required=true since the default value ensures the variable
is always populated.

Co-authored-by: mayor <ec2-user@ip-172-31-43-79.ec2.internal>

.beads/formulas/mol-session-gc.formula.toml

@@ -246,5 +246,4 @@ Dog returns to available state in the pool.
 [vars]
 [vars.mode]
 description = "GC mode: 'conservative' or 'aggressive'"
-required = true
 default = "conservative"

internal/formula/formulas/mol-session-gc.formula.toml

@@ -246,5 +246,4 @@ Dog returns to available state in the pool.
 [vars]
 [vars.mode]
 description = "GC mode: 'conservative' or 'aggressive'"
-required = true
 default = "conservative"