feat(w9d): kei-spawn drive subcommand + AnthropicDriver trait stub
kei-spawn drive <task.toml> internally calls spawn pipeline, emits
SpawnOutput JSON to stdout, returns exit 64 (NotImplemented) from
ManualDriver with stderr instruction to use manual Agent-tool path.
AnthropicDriver trait: ManualDriver (current) + HttpDriver (future
reqwest+tokio+KEI_ANTHROPIC_KEY). Extensibility preserved without
breaking-change deps.
Tests: 10/10 (was 6, +4: 2 drive unit + 2 drive_smoke binary integration).
Exit code contract: 0 success, 1 spawn-fail, 2 verify-fail, 64
NotImplemented (matches kei-runtime::invoke NotImplemented convention).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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