spawn_agent forces approval_policy=never; breaks collab thread navigation and blocks sandbox escalation

Problem

spawn_agent (thread-spawned subagents used by Collab / multi-agent TUI) currently clamps the spawned thread's approval_policy to never.

This causes two practical breakages:
1) Config mismatch errors when navigating/resuming subagent threads (e.g. in the TUI /agents view) if the parent session is started with -a on-request (or any non-never policy). The error looks like:

• OnRequest is not in the allowed set [Never]

2) Spawned agents can't request sandbox escalation approvals (e.g. running Playwright on macOS in workspace-write sandbox often requires escalation to launch Chromium/WebKit/Firefox). With approval_policy=never, the agent can never ask, which forces users to switch to danger-full-access as a workaround.

Root cause

In codex-rs/core/src/tools/handlers/multi_agents.rs, apply_spawn_agent_overrides does:

config.permissions.approval_policy = Constrained::allow_only(AskForApproval::Never);

So the spawned thread is permanently constrained to never even when the parent session is interactive and explicitly configured to use on-request.

Expected

Spawned threads should inherit the parent session's effective approval policy (and constraints) by default.

Non-interactive exec already uses approval_policy=never, so inheriting it keeps the current behavior for headless runs.

Actual

Spawned threads are forced to never, producing the mismatch error above and preventing escalation approvals.

Environment / repro

• Codex CLI: 0.101.0+ (observed in 0.104.0 and main)
• macOS (arm64)
• Start Codex TUI with --full-auto (or -a on-request), spawn an agent, then navigate to the spawned thread via /agents.

Proposed fix

Remove the hard override and let the spawned thread inherit the parent approval policy.

(Background subagents created by other systems can continue to clamp approval_policy=never if needed.)