[codex] Add configurable multi-agent mode hint text (#30493)

## Why

Multi-agent V2 normally derives its mode instructions from reasoning
effort: Ultra enables proactive delegation, while other efforts require
an explicit request. Some deployments need to provide one configured
delegation policy that replaces those built-ins and remains stable when
reasoning effort changes.

## What changed

- Add `features.multi_agent_v2.multi_agent_mode_hint_text` alongside the
existing root and subagent hint settings.
- Treat any configured value, including an empty string, as
`MultiAgentMode::Custom(hint_text)`, so the configured text replaces the
built-in explicit-only and proactive policies.
- Persist the full custom variant and hint text in the turn-context
snapshot, so the durable comparison baseline detects both
reasoning-effort changes and configured policy-text changes.
- Preserve the existing explicit-only/proactive behavior when the
setting is absent.
- Replace the ambiguous `MultiAgentMode::None` variant with
`MultiAgentMode::Custom(String)` in new rollouts and API schemas. A
compatibility wire type maps legacy serialized `none` values to
`Custom("")` when resuming existing rollouts.
- Regenerate the config and app-server schemas.

## Configuration examples

The distinction is whether `multi_agent_mode_hint_text` is present. An
empty string is still a configured value and intentionally suppresses
the built-in mode instructions.

### Unset: preserve existing effort-derived behavior

```toml
[features.multi_agent_v2]
enabled = true
# multi_agent_mode_hint_text is omitted
```

- Ultra reasoning uses the built-in proactive delegation instructions.
- Other reasoning efforts use the built-in explicit-request-only
instructions.

### Empty: suppress all mode hint text

```toml
[features.multi_agent_v2]
enabled = true
multi_agent_mode_hint_text = ""
```

This selects effective mode `custom` at every reasoning effort and
injects an empty mode body, suppressing both built-in policies.

### Set: always use the configured text

```toml
[features.multi_agent_v2]
enabled = true
multi_agent_mode_hint_text = "Delegate to subagents when it will materially improve the result."
```

This selects effective mode `custom` at every reasoning effort and
injects the configured text verbatim instead of either built-in policy.

## Verification

- `just test -p codex-core multi_agent_mode`
- Covers a configured hint across High and Ultra reasoning efforts and
verifies the full custom hint is recorded for both turns.
- Covers an empty-string override suppressing both built-in instruction
bodies.
- `just test -p codex-protocol -p codex-app-server-protocol`
- Covers legacy `none` turn-context deserialization as `Custom("")` and
verifies the regenerated schemas.

codex-rs/features/src/tests.rs

@@ -610,6 +610,7 @@ usage_hint_enabled = false
 usage_hint_text = "Custom delegation guidance."
 root_agent_usage_hint_text = "Root guidance."
 subagent_usage_hint_text = "Subagent guidance."
+multi_agent_mode_hint_text = "Custom mode guidance."
 tool_namespace = "agents"
 hide_spawn_agent_metadata = true
 non_code_mode_only = true
@@ -633,6 +634,7 @@ non_code_mode_only = true
             usage_hint_text: Some("Custom delegation guidance.".to_string()),
             root_agent_usage_hint_text: Some("Root guidance.".to_string()),
             subagent_usage_hint_text: Some("Subagent guidance.".to_string()),
+            multi_agent_mode_hint_text: Some("Custom mode guidance.".to_string()),
             tool_namespace: Some("agents".to_string()),
             hide_spawn_agent_metadata: Some(true),
             non_code_mode_only: Some(true),