Agents.*: depth-1 CLI subagents land in the kernel

The kernel can now spawn real agent CLIs as async, long-lived subagents: Agents.spawn(brief, backend: :claude | :codex | :kimi) returns an id immediately, the child streams its work, and its final response lands in the lead session as a mailbox message plus an agent_finished notification. This is the Fable 5 system card’s top-scoring multi-agent harness (sec 8.15.3, async subagents: BrowseComp 93.3 vs 88.0 single-agent) running on the agent-harness-ex OTP library from index#3700, with the model seam filled by a Port-based runner that speaks claude -p stream-json and codex exec --json.

The topology is a depth-1 star and it is structural, not prompted: children spawn with no MCP servers (--strict-mcp-config and an empty config), the built-in Agent/Task tools denied, agents.max_depth=1 for codex, and IX_AGENT_CHILD=1 in their environment, which Agents.spawn refuses under. There is no child-to-child call in the surface at all. Kimi K3 rides the claude harness pointed at Moonshot’s Anthropic-compatible endpoint.

Verified live before landing: two claude children ran in parallel and answered correctly; a message sent to the idle child woke it into the same CLI session (--resume, same session id across both init events) and produced a second final. Messages queue in the harness and inject after the recipient’s next tool result, the card’s delivery rule.

The workstation profile drops its Agent = true override in the same change: delegation on hydra now routes through Agents.* instead of the harness Task tool, which is what the card’s data says to do (the async-subagent hierarchy beat the peer mesh on score and the blocking orchestrator on everything).