Adds FleetExecutor: drives a worker as a local `codewhale exec` subprocess via
the existing host adapter, incrementally drains its stream-json output into
FleetWorkerEventPayload ledger events, and finalizes the terminal outcome from
the process exit. The worker's heavy runtime/tool construction lives in its own
process, so the orchestrator only ingests a compact event stream — the
isolation pattern that keeps fanout light (per Codex/Kimi/Claude Code).
Verified end-to-end by an integration test that runs a REAL subprocess emitting
stream-json (standing in for `codewhale exec`) through the real adapter and
asserts RunningTool + terminal Completed events flow out — no codewhale binary
needed. 8 executor tests pass; the 58 existing fleet tests stay green
(executor is not yet wired into `codewhale fleet run`, so no behavior change).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Hunter B
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