Handle "Don't Trust" directory selection in onboarding (#4941)

Fixes #4940
Fixes #4892

When selecting "No, ask me to approve edits and commands" during
onboarding, the code wasn't applying the correct approval policy,
causing Codex to block all write operations instead of requesting
approval.

This PR fixes the issue by persisting the "DontTrust" decision in
config.toml as `trust_level = "untrusted"` and handling it in the
sandbox and approval policy logic, so Codex correctly asks for approval
before making changes.

## Before (bug)
<img width="709" height="500" alt="bef"
src="https://github.com/user-attachments/assets/5aced26d-d810-4754-879a-89d9e4e0073b"
/>

## After (fixed)
<img width="713" height="359" alt="aft"
src="https://github.com/user-attachments/assets/9887bbcb-a9a5-4e54-8e76-9125a782226b"
/>

---------

Co-authored-by: Eric Traut <etraut@openai.com>

codex-rs/protocol/src/config_types.rs

@@ -98,3 +98,13 @@ pub enum ForcedLoginMethod {
     Chatgpt,
     Api,
 }
+
+/// Represents the trust level for a project directory.
+/// This determines the approval policy and sandbox mode applied.
+#[derive(Debug, Serialize, Deserialize, Clone, Copy, PartialEq, Eq, Display, JsonSchema, TS)]
+#[serde(rename_all = "lowercase")]
+#[strum(serialize_all = "lowercase")]
+pub enum TrustLevel {
+    Trusted,
+    Untrusted,
+}