multi_agent_v2: move thread cap into feature config

[jif-oai]

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.

[jif-oai]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 467e4641ee

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".