## Summary
Multi-agent v2 tools now use the fixed `collaboration` namespace when
namespace tools are available. This keeps the model-visible hint and the
actual tool surface aligned around `functions.collaboration.*`, without
exposing an unshipped namespace knob to users.
The PR also removes the old `features.multi_agent_v2.tool_namespace`
config/schema surface, updates the MAv2 test fixtures for namespaced
calls, and fixes stale `TurnContext.features` references that were
breaking `codex-core` builds.
## Changes
- Expose MAv2 tools under `collaboration` instead of relying on a
configurable namespace.
- Remove `tool_namespace` from MAv2 TOML config, resolved config,
validation, schema, and tests.
- Update tool-planning and integration fixtures to assert or emit
namespaced MAv2 tool calls.
- Read feature state through `TurnContext.config.features` in the
multi-agent mode context paths.
## Testing
- `just write-config-schema`
- `just test -p codex-features`
## Why
Multi-Agent V2 concurrency should count active non-root turns, not
resident or durable agent threads. The limit is intentionally best
effort: admission checks are synchronous, but concurrent successful
checks may overshoot slightly.
## What changed
- Keep one root-derived execution limit on the shared `AgentControl`.
- Count active V2 subagent turns with an RAII guard owned by
`RunningTask`.
- Check capacity before spawning or starting an idle agent, including
direct app-server `turn/start` submissions.
- Preserve queued delivery for agents that are already running.
- Exempt automatic idle continuations so `/goal` work is not dropped
when capacity is temporarily full.
- Keep root and V1 turns outside this limiter.
## Test coverage
- `execution_guards_count_active_v2_subagent_turns`
- `execution_guards_ignore_root_and_v1_turns`
- `v2_nested_spawn_checks_shared_active_execution_capacity`
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