Feature: Background subagents should be able to escalate permission requests to the parent session

Problem

When a background subagent (dispatched via Agent tool with run_in_background: true) encounters a tool call that isn't pre-approved in settings.json, it silently auto-denies. The subagent fails without any way to request the missing permission from the user.

This makes background subagents fragile — if you miss even one permission in your pre-approved list, the subagent silently fails and loses all its work.

Current Behavior

1. Parent dispatches background subagent
2. Subagent needs Bash(docker compose up) which isn't pre-approved
3. Tool call is auto-denied silently
4. Subagent fails or produces incomplete output
5. User has no idea a permission was needed until they read the error

Desired Behavior

1. Parent dispatches background subagent
2. Subagent needs Bash(docker compose up) which isn't pre-approved
3. Subagent pauses and raises a permission request to the parent session
4. Parent session shows the user:

``
Background agent "build-service" needs permission:
Bash: docker compose up
[Approve once] [Approve for session] [Deny]
``

1. User approves → permission injected → subagent resumes
2. User denies → subagent informed, can handle gracefully

Use Case

Any orchestration pattern that dispatches multiple background subagents in parallel. Pre-approving every possible permission upfront works for 90% of cases, but the remaining 10% — where an agent discovers it needs something unexpected at runtime — currently causes silent failures.

Related Issues

• #23983 — PermissionRequest hooks not triggered for subagent requests (prerequisite)
• #27203 — canUseTool callback not invoked for background subagent tool calls (prerequisite)
• #32402 — Background subagents silently auto-deny permissions (this is the symptom)

Both #23983 and #27203 are prerequisites for this — hooks and callbacks need to fire before escalation can work. But even with those fixed, there's no mechanism for a background subagent to pause execution and wait for a permission decision from the parent.

Suggested Approach

• Background subagent enters a "waiting for permission" state instead of auto-denying
• Parent session is notified (similar to how task completion notifications work)
• User approves/denies via the parent session
• Subagent resumes or handles denial gracefully
• Approved permissions accumulate for the rest of the session (don't ask twice)