Add a `spawn` action to the /subagents command handler that invokes
spawnSubagentDirect() to deterministically launch a named subagent.
Usage: /subagents spawn <agentId> <task> [--model <model>] [--thinking <level>]
Also includes the shared subagent-spawn module extraction (same as the
refactor/extract-shared-subagent-spawn branch) since it hasn't merged yet.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
