🤖 refactor: simplify CI and tests folder structure (#908)

## Summary

Simplify CI workflow and reorganize tests folder for cleaner filtering.

## Tests Folder Reorganization

```
tests/
├── setup.ts         # Global Jest setup
├── testUtils.ts     # Shared utilities
├── __mocks__/       # Jest mocks
├── ipc/             # Backend IPC integration tests (moved from integration/ipc/)
├── runtime/         # Runtime tests (Docker SSH)
├── e2e/             # Playwright E2E tests
└── worker-test.test.ts
```

**Philosophy:** Unit tests colocated in `src/`, integration tests in
`tests/`.

- Flattened `tests/integration/ipc/` → `tests/ipc/`
- Moved `tests/models/knownModels.test.ts` → `src/common/constants/`
(colocated with implementation)

## CI Integration Test Filtering

| Change Type | Integration Tests |
|-------------|-------------------|
| Docs-only | Skip entire job |
| Browser-only | Run `tests/` excluding `tests/ipc/` |
| Otherwise | Run all `tests/` |

**Future-proof:** New `tests/foo/*.test.ts` directories are
automatically included. Uses `--testPathIgnorePatterns` for browser-only
exclusion instead of hardcoding paths.

## Other Changes

- Runtime tests now actually run in CI (were previously missed)
- Backend filter uses `tests/**` with `!tests/e2e/**` exclusion

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_Generated with `mux`_

Author: ammar-agent