Add per-turn multi-agent mode (#28685)

## Why

Multi-agent v2 currently carries an explicit-request-only delegation
rule in its static usage hint. That provides a safe default, but it
prevents clients from selecting proactive delegation per turn without
changing static guidance or rewriting prior model context.

This change makes delegation mode a session selection that can be
updated through `turn/start`, while deriving the effective model-visible
mode separately for each turn. Eligible multi-agent v2 turns remain
explicit-request-only unless proactive mode is both selected and
enabled.

## What changed

- Add the experimental `turn/start.multiAgentMode` parameter with
`explicitRequestOnly` and `proactive` values. Omission retains the
loaded session's current optional selection.
- Add the default-off `features.multi_agent_mode` feature gate. Eligible
multi-agent v2 turns use the selected mode when enabled; an unset
selection or disabled gate resolves to `explicitRequestOnly`.
- Treat mode prompting as inapplicable for multi-agent v1 and other
unsupported session configurations, producing no multi-agent mode
developer message rather than rejecting the turn.
- Move the explicit-request-only rule out of the static v2 usage hint
and into a bounded, tagged developer context fragment.
- Emit the effective mode in initial context and only when that
effective mode changes on later turns.
- Persist the effective mode in `TurnContextItem` as the durable
baseline for resume and context-update comparisons.

Historical rollout items are not rewritten. Later mode developer
messages establish the current rule incrementally.

## Not covered

- Initial selection through `thread/start` and selected-mode reporting
from thread lifecycle/settings APIs; those are isolated in the stacked
#28792.
- A TUI control or slash command for selecting the mode.
- Persisting a preferred mode to `config.toml`; selection remains
session/turn scoped.
- Changes to multi-agent concurrency limits, tool availability, or model
catalog capability declarations.
- Rewriting historical rollout prompt items. Cold resume restores the
latest persisted effective mode when available while leaving historical
developer messages intact.

## Verification

- `CARGO_INCREMENTAL=0 just test -p codex-core multi_agent_mode`
- Focused app-server coverage verifies that `turn/start.multiAgentMode`
produces proactive developer instructions for an eligible v2 turn.

## Stack

Followed by #28792, which adds `thread/start` initialization and
lifecycle/settings observability.

codex-rs/core/src/config/config_tests.rs

@@ -10055,9 +10055,8 @@ max_concurrent_threads_per_session = 17
 
     let config = resolve_multi_agent_v2_config(&config_toml);
     let concurrency_guidance = "There are 17 available concurrency slots, meaning that up to 17 agents can be active at once, including you.";
-    let expected_suffix = format!(
-        "{DEFAULT_MULTI_AGENT_V2_SHARED_USAGE_HINT_TEXT}\n{concurrency_guidance}\n\n{DEFAULT_MULTI_AGENT_V2_NO_SPAWN_HINT_TEXT}"
-    );
+    let expected_suffix =
+        format!("{DEFAULT_MULTI_AGENT_V2_SHARED_USAGE_HINT_TEXT}\n{concurrency_guidance}");
     assert!(
         [
             config.root_agent_usage_hint_text,

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

@@ -247,11 +247,9 @@ All agents share the same directory. In detail:
 - All agents use the same current working directory.
 - As a result, edits made by one agent are immediately visible to all other agents.
 "#;
-const DEFAULT_MULTI_AGENT_V2_NO_SPAWN_HINT_TEXT: &str = "Do not spawn sub-agents unless the user explicitly asks for sub-agents, delegation, or parallel agent work.";
-
 fn default_multi_agent_v2_usage_hint_text(usage_hint_text: &str, max_concurrency: usize) -> String {
     format!(
-        "{usage_hint_text}\n{DEFAULT_MULTI_AGENT_V2_SHARED_USAGE_HINT_TEXT}\nThere are {max_concurrency} available concurrency slots, meaning that up to {max_concurrency} agents can be active at once, including you.\n\n{DEFAULT_MULTI_AGENT_V2_NO_SPAWN_HINT_TEXT}"
+        "{usage_hint_text}\n{DEFAULT_MULTI_AGENT_V2_SHARED_USAGE_HINT_TEXT}\nThere are {max_concurrency} available concurrency slots, meaning that up to {max_concurrency} agents can be active at once, including you."
     )
 }