fix(task): preserve subagent self permissions

[kitlangton]

Summary

• Preserve Plan Mode's inherited edit restriction for subagents.
• Stop inheriting unrelated parent-agent self-denies like read, bash, and task into delegated subagents.
• Add regression coverage for controller/executor deny-by-default delegation and parent session deny ceilings.

Test

• bun run test test/permission-task.test.ts test/agent/plan-mode-subagent-bypass.test.ts
• bun typecheck
• push hook: bun turbo typecheck

Fixes #26700
Fixes #26747
Fixes #26758
Fixes #27123

[Sewer56]

@kitlangton This PR introduces a regression.

Parent agents with scoped edit permissions like:

edit:
  "*": deny
  "specific-pattern": allow

now break subagents that have their own scoped edit allows:

edit:
  "other-pattern": allow

other-pattern can no longer be accessed.

Merging of permissions puts parent's "*": deny last, overwriting any allow rules by child.