refactor: domain folder layout (types, API, core, engine) (#2398)
## Summary
Wave 17 organizes Fusion into **domain folders** (stacks on #2397).
### Layout
- **core/types/** — board, task, agents, settings, merge, workflow,
mesh, …
- **core/src/** — agents, ai, async-stores, workflows, tasks, config,
db, …
- **dashboard/app/api/** — client, tasks, agents, git, missions,
planning, …
- **engine/src/** — agents, auth, execution, merge, missions, overseer,
worktree, …
Root keepers retained for large entrypoints (`store.ts`, `executor.ts`,
`merger.ts`, …).
Public barrels (`@fusion/core`, `@fusion/engine`, `app/api.ts` → legacy)
stay stable.
## Test plan
- [x] `@fusion/core` typecheck
- [x] `@fusion/engine` typecheck (pre-existing playwright-core noise
only)
- [ ] CI merge gate
**Stack:** #2394 → #2397 → **this PR**
cb57093d03