SendMessage tool unavailable without agent teams feature, breaking agent resume

Bug

The SendMessage tool (used to resume/continue a previously spawned subagent) is now gated behind the "agent teams" feature. This is a regression — SendMessage is required for basic agent communication, not just for multi-agent team coordination.

Impact

Without SendMessage, there is no way to continue a conversation with a previously spawned agent. The Agent tool's result text still instructs users to "use SendMessage with to: '<agentId>' to continue this agent," but the tool is not available.

Example: You spawn an agent and ask it "what is 1+1?" — it answers "2." You then want to follow up with "how hard was that question to answer on a scale of 1-10?" — you can't, because SendMessage is unavailable. Spawning a new agent loses all prior context.

Repro

1. Use an account without access to the "agent teams" feature
2. Spawn an agent via the Agent tool — note the returned agentId
3. Try to find SendMessage via ToolSearch — it does not appear in primary or deferred tools
4. Attempt to resume the agent — fails, spawns a new agent with no history

Expected

SendMessage should be available to all users as a core tool for resuming subagents, independent of the agent teams feature gate.

Actual

SendMessage is only available when agent teams is enabled. Users without the feature cannot resume any subagent, despite the system instructing them to do so.

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