Adds docs/AGENT_RUNTIME.md: one headless runtime, three launchers (TUI turn,
`codewhale exec`, Agent Fleet); sub-agent == fleet worker == nested/launched run
of the same runtime; the isolation+events+compact-projection lag fix; the single
recursion axis; and the one event vocabulary. Answers #2972 (how much Claude
Code convergence): converge on shape (headless runtime, isolated sub-agents,
event-driven fanout, skills, receipts), keep CodeWhale branding + multi-provider
+ the local-first fleet distinct, and never fork execution semantics per surface.
Cross-linked from FLEET.md.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Hunter B
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