The "agent-neutral codebase" framing is the right abstraction. We ended up building a small generator that takes a single spec file and emits the agent-specific config (CLAUDE.md, AGENTS.md, .cursor/rules) rather than maintaining symlinks. Easier to version and to add a new agent when they inevitably ship next month.

The pain point you're underselling is hooks. They're the least portable piece by far because each harness has its own event model. Skills port reasonably, subagents mostly port, hooks almost never do.