feat(core) Introduce Feature::RequestPermissions (#11871)

## Summary
Introduces the initial implementation of Feature::RequestPermissions.
RequestPermissions allows the model to request that a command be run
inside the sandbox, with additional permissions, like writing to a
specific folder. Eventually this will include other rules as well, and
the ability to persist these permissions, but this PR is already quite
large - let's get the core flow working and go from there!

<img width="1279" height="541" alt="Screenshot 2026-02-15 at 2 26 22 PM"
src="https://github.com/user-attachments/assets/0ee3ec0f-02ec-4509-91a2-809ac80be368"
/>

## Testing
- [x] Added tests
- [x] Tested locally
- [x] Feature

codex-rs/core/src/unified_exec/mod.rs

@@ -29,6 +29,7 @@ use std::sync::Weak;
 use std::time::Duration;
 
 use codex_network_proxy::NetworkProxy;
+use codex_protocol::models::AdditionalPermissions;
 use rand::Rng;
 use rand::rng;
 use tokio::sync::Mutex;
@@ -89,6 +90,7 @@ pub(crate) struct ExecCommandRequest {
     pub network: Option<NetworkProxy>,
     pub tty: bool,
     pub sandbox_permissions: SandboxPermissions,
+    pub additional_permissions: Option<AdditionalPermissions>,
     pub justification: Option<String>,
     pub prefix_rule: Option<Vec<String>>,
 }
@@ -229,6 +231,7 @@ mod tests {
                     network: None,
                     tty: true,
                     sandbox_permissions: SandboxPermissions::UseDefault,
+                    additional_permissions: None,
                     justification: None,
                     prefix_rule: None,
                 },

codex-rs/core/src/unified_exec/process_manager.rs

@@ -590,6 +590,7 @@ impl UnifiedExecProcessManager {
             network: request.network.clone(),
             tty: request.tty,
             sandbox_permissions: request.sandbox_permissions,
+            additional_permissions: request.additional_permissions.clone(),
             justification: request.justification.clone(),
             exec_approval_requirement,
         };