score/time_slave: Create AoU requirements

> [!IMPORTANT]
> Link the implementing PR to this issue by including `Closes #82` in the PR description or commit message.

### What

Write the Assumption of Use requirements for `score/time_slave`, capturing all deployment and runtime preconditions that integrators must fulfil.

`score/time_slave` runs as a service alongside the gPTP daemon. Examples: the gPTP daemon must be running and reachable before TimeSlave starts; PHC device paths must be accessible with the correct permissions; platform-specific resource limits (e.g. real-time thread priorities on QNX) must be met by the deployment environment. Author as `aou_req` nodes at `score/time_slave/docs/requirements/time_slave_aou.rst`.

### Acceptance Criteria

- [ ] `score/time_slave/docs/requirements/time_slave_aou.rst` created
- [ ] All deployment and runtime preconditions captured as `aou_req` nodes with `:status: valid`
- [ ] Every `aou_req` links back to the relevant `comp_req` it restricts

### References

- [S-CORE Requirements Engineering](https://eclipse-score.github.io/process_description/main/process_areas/requirements_engineering/)