Per-provider default permission mode, applied when switching models

[ericrcwu001]

Summary

Let me set a default permission mode (runtime mode) for each provider. When I switch a thread or the composer to a model from that provider, T3 Code should switch to that mode automatically.

My case: I want Claude threads in Full access and Codex threads in Auto. Today I have to remember to change the mode by hand every time I pick a Codex model.

Current behavior (0.0.46-nightly.20261007)

• defaultRuntimeMode (added after Allow configuring the default runtime mode #7028) is one setting for the whole environment. It can be overridden per project, but not per provider or model.
• Switching models keeps the thread's current runtime mode. If I start in Full access with Claude and switch to Codex, Codex runs with approvalPolicy: "never" and sandboxPolicy: dangerFullAccess.
• Providers already declare supportedRuntimeModes, so the app knows which modes each provider supports. It just has no per-provider preference.

Proposed behavior

1. Setting: add an optional defaultRuntimeMode to each provider instance in Settings → Providers. Example:

   "providerInstances": {
     "codex":      { "driver": "codex", "defaultRuntimeMode": "auto", ... },
     "codexAlpha": { "driver": "codex", "defaultRuntimeMode": "auto", ... }
   }

   If it's unset, keep today's behavior: fall back to the project default, then the environment default.

2. On model switch: when the selected model moves to a different provider instance that has a defaultRuntimeMode set, change the composer's or thread's runtime mode to that value.

   • Only do this when the provider instance changes, not when switching between models of the same provider.
   • Show a short toast or inline note, e.g. "Switched to Auto (Codex default)", so the change is visible and easy to undo.
   • If I change the mode by hand afterwards, keep my choice for that thread until the provider changes again.

3. New threads: for a new thread, the mode should resolve in this order: provider default → project default → environment default.

Design questions

• Precedence vs. project override: should a project's defaultRuntimeMode beat the provider default? I'd suggest the provider default wins on a model switch, and the project default applies when no provider default is set.
• Delegated tasks and subagents: the escalation rule (a child can't be broader than its parent) should still apply. A provider default should only ever narrow a child's mode, never widen it.
• Mid-session switching: for providers where supportsRuntimeModeSwitchInSession is false, apply the new mode on the next session or turn, the same way a manual mode change works today.

Why

Providers handle permission modes differently. With Claude, Full access is a reasonable default for me. With Codex, Full access turns off both the approval prompts and the sandbox, and Auto (on-request approvals with the auto-reviewer, workspace-write sandbox) is a better default. One global mode makes users pick between the two, or remember to flip it every time they switch models, which is easy to forget.