Harden TUI subagent navigation (#39088)

## What changed

- Use `/subagents` consistently for the subagent picker and status UI, removing the `/agent` alias.
- Rejoin already-loaded subagent threads without overriding their existing settings.
- Route notifications and approval requests only to the active thread or its known subagents, discard unrelated startup events, and avoid replaying resolved approvals.

## Testing

- Add coverage for filtering buffered startup events, dropping resolved approvals, preserving owned subagent approvals, and resuming threads without setting overrides.

GitOrigin-RevId: b4e597f7cd4756faccf31d8b964e7a172f5ab0eb

codex-rs/tui/src/slash_command.rs

@@ -40,7 +40,6 @@ pub enum SlashCommand {
     Compact,
     Plan,
     Goal,
-    Agent,
     Side,
     Btw,
     Copy,
@@ -127,7 +126,7 @@ impl SlashCommand {
             SlashCommand::Personality => "choose a communication style for Codex",
             SlashCommand::Plan => "switch to Plan mode",
             SlashCommand::Goal => "set or view the goal for a long-running task",
-            SlashCommand::Agent | SlashCommand::MultiAgents => "switch the active agent thread",
+            SlashCommand::MultiAgents => "switch between this session's subagents",
             SlashCommand::Side | SlashCommand::Btw => {
                 "start a side conversation in an ephemeral fork"
             }
@@ -256,7 +255,7 @@ impl SlashCommand {
             | SlashCommand::Btw => true,
             SlashCommand::Rollout => true,
             SlashCommand::TestApproval => true,
-            SlashCommand::Agent | SlashCommand::MultiAgents => true,
+            SlashCommand::MultiAgents => true,
             SlashCommand::Theme | SlashCommand::Pets => false,
         }
     }