Simplify mode permissions: collapse auto_approve into approval_mode (4 knobs → 2), keep trust_mode for workspace-write only

[Hmbown]

Summary

The mode/permission model carries four overlapping knobs in EffectiveModePolicy (crates/tui/src/tui/app.rs:1110-1119): allow_shell, approval_mode, trust_mode, auto_approve. Three of them co-vary in every mode, which is the structural source of the "UI says one mode but a different flag decides" bugs (#3733 Auto, #3735 YOLO-publish).

Current state (3 surviving modes after Auto deferral)

Mode	allow_shell	approval_mode	trust_mode	auto_approve
Plan	false	Suggest	false	false
Agent	prefs	prefs	prefs	false
YOLO	true	Bypass	true	true

YOLO sets three flags that all mean "approve everything."

What each knob really does

• approval_mode (approval.rs:41-51): Suggest/Auto/Bypass/Never — the one genuinely meaningful control.
• trust_mode (bool): enables sandbox workspace writes, skips the untrusted-workspace safety hold, and contributes to auto-approval (auto_approve || trust_mode, runtime_threads.rs:3296).
• auto_approve (bool): skip prompts; fully covered by approval_mode == Bypass.

Proposal — collapse 4 → 2

1. approval_mode stays the single approve/ask/bypass/never control. Derive auto-approval from approval_mode == Bypass and delete auto_approve (remove the separate flag at app.rs:1117-1118 and its mirrors; replace auto_approve || trust_mode with approval_mode == Bypass || trust_mode).
2. trust_mode keeps its one distinct job — workspace-write capability + untrusted-workspace handling — and stops doubling as an approval flag.
3. allow_shell can likely be derived too (Plan = no shell; Agent/YOLO = shell), but keep for now if prefs need per-user shell toggle in Agent.

Why

Removing auto_approve as an independent flag eliminates the layer split that lets safety_floor's HoldForReview get silently bypassed (#3735) and that makes mode label and behavior disagree (#3733). One control = one source of truth.

Verification

• Existing mode/approval tests still pass after deriving auto-approval from approval_mode.
• Add: YOLO still auto-approves ordinary tools; publish-like HoldForReview still prompts (ties to YOLO mode silently approves publish actions (cargo publish / git push --tags), defeating safety_floor durable-review #3735).

Refs #3733, #3735, #3568.