refactor: package code organization wave 9 (#2274)
## Summary
Wave 9 of package code organization (plan:
`docs/plans/2026-07-14-001-refactor-package-code-organization-plan.md`),
after #2252.
### Peels
| New module | Parent |
|---|---|
| `app/api/global-and-pi-settings.ts` | `legacy.ts` |
| `app/api/task-content.ts` | `legacy.ts`
(attachments/logs/comments/docs/artifacts) |
| `types/messages.ts` | `types.ts` |
| `merger-diff-volume-gate` helpers | `merger.ts` |
Public paths stay stable via re-exports.
### LOC
- `legacy.ts` ~10561 → ~10269
- `types.ts` ~6998 → ~6937
- `merger.ts` ~11223 → ~11208
## Test plan
- [x] core/engine/dashboard typecheck (app)
- [x] eslint on peeled modules
- [x] api-tasks + memory/agent-log focused tests
- [ ] CI merge gate
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **New Features**
* Added dashboard API endpoints for global and project-scoped settings,
PI configuration, notification testing, and PI package
install/reinstall.
* Added task content APIs covering attachments, agent logs, session
files, task comments, task documents, artifacts, and project markdown
files.
* Added structured messaging contracts and participant/message
normalization.
* Added diff-volume gate settings resolution and formatted findings
output.
* **Refactor**
* Organized messaging and dashboard API surface into dedicated modules
via re-exports.
* Removed a deprecated verification-output alias.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
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