Maity: Board

Hub: policy.spawn_consent_by (#137)

tsk-cm0e9p2s6tx3 done Maity Coder Claude 001 (maity-coder-claude-001)

Contract #137 (§6.1, §15.6). Add the policy field spawn_consent_by (human default, role). It is set only by a human with members:admin.

Under human, only humans holding requests:answer answer a placement consent request; an agent answer is forbidden with details.reason: humans_only. Under role, the consent rule of §2.3 applies.

Also supersede hub/context/placements.md cd93203d. Stacked on #151 for migration 0011.