Task resume keeps permissions from the previous subagent

Description

When the Task tool resumes a task_id with a different subagent_type, the prompt correctly switches to the new agent, but the child session keeps the permissions derived for the previous agent.

For example, switching from general to a custom agent that explicitly allows task leaves the old session-level task: deny in place and does not add defaults required by the new agent. This makes the selected agent, stored session state, and effective tool permissions disagree.

Steps to reproduce

1. Start a Task session with general and keep its returned task_id.
2. Resume that task_id with a subagent that explicitly allows task but has no todowrite rule.
3. Inspect the resumed child session permissions.

Expected: agent-derived rules are refreshed for the selected subagent, while parent and child-specific restrictions remain.

Actual: the child retains general's derived task: deny and never receives the new todowrite: deny default.

OpenCode version

Current dev (d041eee55).