Add Smart Approvals guardian review across core, app-server, and TUI (#13860)

## Summary
- add `approvals_reviewer = "user" | "guardian_subagent"` as the runtime
control for who reviews approval requests
- route Smart Approvals guardian review through core for command
execution, file changes, managed-network approvals, MCP approvals, and
delegated/subagent approval flows
- expose guardian review in app-server with temporary unstable
`item/autoApprovalReview/{started,completed}` notifications carrying
`targetItemId`, `review`, and `action`
- update the TUI so Smart Approvals can be enabled from `/experimental`,
aligned with the matching `/approvals` mode, and surfaced clearly while
reviews are pending or resolved

## Runtime model
This PR does not introduce a new `approval_policy`.

Instead:
- `approval_policy` still controls when approval is needed
- `approvals_reviewer` controls who reviewable approval requests are
routed to:
  - `user`
  - `guardian_subagent`

`guardian_subagent` is a carefully prompted reviewer subagent that
gathers relevant context and applies a risk-based decision framework
before approving or denying the request.

The `smart_approvals` feature flag is a rollout/UI gate. Core runtime
behavior keys off `approvals_reviewer`.

When Smart Approvals is enabled from the TUI, it also switches the
current `/approvals` settings to the matching Smart Approvals mode so
users immediately see guardian review in the active thread:
- `approval_policy = on-request`
- `approvals_reviewer = guardian_subagent`
- `sandbox_mode = workspace-write`

Users can still change `/approvals` afterward.

Config-load behavior stays intentionally narrow:
- plain `smart_approvals = true` in `config.toml` remains just the
rollout/UI gate and does not auto-set `approvals_reviewer`
- the deprecated `guardian_approval = true` alias migration does
backfill `approvals_reviewer = "guardian_subagent"` in the same scope
when that reviewer is not already configured there, so old configs
preserve their original guardian-enabled behavior

ARC remains a separate safety check. For MCP tool approvals, ARC
escalations now flow into the configured reviewer instead of always
bypassing guardian and forcing manual review.

## Config stability
The runtime reviewer override is stable, but the config-backed
app-server protocol shape is still settling.

- `thread/start`, `thread/resume`, and `turn/start` keep stable
`approvalsReviewer` overrides
- the config-backed `approvals_reviewer` exposure returned via
`config/read` (including profile-level config) is now marked
`[UNSTABLE]` / experimental in the app-server protocol until we are more
confident in that config surface

## App-server surface
This PR intentionally keeps the guardian app-server shape narrow and
temporary.

It adds generic unstable lifecycle notifications:
- `item/autoApprovalReview/started`
- `item/autoApprovalReview/completed`

with payloads of the form:
- `{ threadId, turnId, targetItemId, review, action? }`

`review` is currently:
- `{ status, riskScore?, riskLevel?, rationale? }`
- where `status` is one of `inProgress`, `approved`, `denied`, or
`aborted`

`action` carries the guardian action summary payload from core when
available. This lets clients render temporary standalone pending-review
UI, including parallel reviews, even when the underlying tool item has
not been emitted yet.

These notifications are explicitly documented as `[UNSTABLE]` and
expected to change soon.

This PR does **not** persist guardian review state onto `thread/read`
tool items. The intended follow-up is to attach guardian review state to
the reviewed tool item lifecycle instead, which would improve
consistency with manual approvals and allow thread history / reconnect
flows to replay guardian review state directly.

## TUI behavior
- `/experimental` exposes the rollout gate as `Smart Approvals`
- enabling it in the TUI enables the feature and switches the current
session to the matching Smart Approvals `/approvals` mode
- disabling it in the TUI clears the persisted `approvals_reviewer`
override when appropriate and returns the session to default manual
review when the effective reviewer changes
- `/approvals` still exposes the reviewer choice directly
- the TUI renders:
- pending guardian review state in the live status footer, including
parallel review aggregation
  - resolved approval/denial state in history

## Scope notes
This PR includes the supporting core/runtime work needed to make Smart
Approvals usable end-to-end:
- shell / unified-exec / apply_patch / managed-network / MCP guardian
review
- delegated/subagent approval routing into guardian review
- guardian review risk metadata and action summaries for app-server/TUI
- config/profile/TUI handling for `smart_approvals`, `guardian_approval`
alias migration, and `approvals_reviewer`
- a small internal cleanup of delegated approval forwarding to dedupe
fallback paths and simplify guardian-vs-parent approval waiting (no
intended behavior change)

Out of scope for this PR:
- redesigning the existing manual approval protocol shapes
- persisting guardian review state onto app-server `ThreadItem`s
- delegated MCP elicitation auto-review (the current delegated MCP
guardian shim only covers the legacy `RequestUserInput` path)

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/protocol/src/approvals.rs

@@ -84,6 +84,23 @@ pub enum NetworkPolicyRuleAction {
     Deny,
 }
 
+#[derive(Debug, Clone, Copy, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "lowercase")]
+pub enum GuardianRiskLevel {
+    Low,
+    Medium,
+    High,
+}
+
+#[derive(Debug, Clone, Copy, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "snake_case")]
+pub enum GuardianAssessmentStatus {
+    InProgress,
+    Approved,
+    Denied,
+    Aborted,
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
 pub struct NetworkPolicyAmendment {
     pub host: String,
@@ -97,6 +114,35 @@ pub struct ExecApprovalRequestSkillMetadata {
     pub path_to_skills_md: PathBuf,
 }
 
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, JsonSchema, TS)]
+pub struct GuardianAssessmentEvent {
+    /// Stable identifier for this guardian review lifecycle.
+    pub id: String,
+    /// Turn ID that this assessment belongs to.
+    /// Uses `#[serde(default)]` for backwards compatibility.
+    #[serde(default)]
+    pub turn_id: String,
+    pub status: GuardianAssessmentStatus,
+    /// Numeric risk score from 0-100. Omitted while the assessment is in progress.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    #[ts(optional)]
+    pub risk_score: Option<u8>,
+    /// Coarse risk label paired with `risk_score`. Omitted while in progress.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    #[ts(optional)]
+    pub risk_level: Option<GuardianRiskLevel>,
+    /// Human-readable explanation of the final assessment. Omitted while in progress.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    #[ts(optional)]
+    pub rationale: Option<String>,
+    /// Canonical action payload that was reviewed. Included when available so
+    /// clients can render pending or resolved review state alongside the
+    /// reviewed request.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    #[ts(optional)]
+    pub action: Option<JsonValue>,
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]
 pub struct ExecApprovalRequestEvent {
     /// Identifier for the associated command execution item.