multi_agent_v2: move thread cap into feature config (#19792)

## Why

`features.multi_agent_v2.max_concurrent_threads_per_session` is meant to
be the MultiAgentV2-specific session thread cap: it counts the root
thread and all open subagent threads. The previous implementation kept
this surface tied to `agents.max_threads`, which made it a global
subagent-only cap and allowed the legacy setting to coexist with
MultiAgentV2.

## What Changed

- Added `max_concurrent_threads_per_session` to
`[features.multi_agent_v2]` with default `4`.
- Removed the `[agents] max_concurrent_threads_per_session` alias to
`agents.max_threads`.
- When MultiAgentV2 is enabled, reject `agents.max_threads` and derive
the existing internal subagent slot limit as
`max_concurrent_threads_per_session - 1`.
- Regenerated `core/config.schema.json` and added coverage for the new
config semantics.

## Result
```
➜  codex git:(jif/clean-multi-agent-v2-config) codex -c features.multi_agent_v2.enabled=true -c features.multi_agent_v2.max_concurrent_threads_per_session=3
╭────────────────────────────────────────────────────╮
│ >_ OpenAI Codex (v0.0.0)                           │
│                                                    │
│ model:     gpt-5.5 xhigh   fast   /model to change │
│ directory: ~/code/codex                            │
╰────────────────────────────────────────────────────╯

  Tip: Update Required - This version will no longer be supported starting May 8th. Please upgrade to the latest version (https://github.com/openai/codex/releases/latest) using your preferred package manager.

› Can you try to spawn 4 agents


• I’ll try to start four lightweight agents at once and report exactly what the runtime accepts.

• Spawned Russell [no-apps] (gpt-5.5 xhigh)
  └ Spawn probe 1: reply briefly that you started, then wait for further instructions. Do not do any repo work.

• Spawned Descartes [no-apps] (gpt-5.5 xhigh)
  └ Spawn probe 2: reply briefly that you started, then wait for further instructions. Do not do any repo work.

• Agent spawn failed
  └ Spawn probe 3: reply briefly that you started, then wait for further instructions. Do not do any repo work.

• Agent spawn failed
  └ Spawn probe 4: reply briefly that you started, then wait for further instructions. Do not do any repo work.

• The runtime accepted the first two and rejected the next two with agent thread limit reached. I’m checking whether the two accepted probes have returned cleanly, then I’ll close them if needed.
```

---------

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

codex-rs/core/config.schema.json

@@ -1310,6 +1310,11 @@
         "hide_spawn_agent_metadata": {
           "type": "boolean"
         },
+        "max_concurrent_threads_per_session": {
+          "format": "uint",
+          "minimum": 1.0,
+          "type": "integer"
+        },
         "usage_hint_enabled": {
           "type": "boolean"
         },