The ACP agent was spawned with only `kill_on_drop`, which SIGKILLs the
agent's own pid. Commands the agent had started (a `nix build`, a test
run) were reparented and kept running after the agent was killed: when a
subagent run ends after `interrupt`, when an agent ignores
`session/cancel` past the grace, and when the harness drops the runtime.
The agent now leads its own process group (`process_group(0)`), and
dropping its `Connection` SIGKILLs that group. The soft path is unchanged:
`interrupt` / `Canceller::cancel` still sends `session/cancel`.
Refs #4999. The claude half needs hive-claude 0.1.3, which is not on
crates.io yet.
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