When sessions_spawn is called without runTimeoutSeconds, subagents
previously defaulted to 0 (no timeout). This adds a config key at
agents.defaults.subagents.runTimeoutSeconds so operators can set a
global default timeout for all subagent runs.
The agent-provided value still takes precedence when explicitly passed.
When neither the agent nor the config specifies a timeout, behavior is
unchanged (0 = no timeout), preserving backwards compatibility.
Updated for the subagent-spawn.ts refactor (logic moved from
sessions-spawn-tool.ts to spawnSubagentDirect).
Closes#19288
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
