feat(prompt): support per-island system messages

[Mqw7373]

Summary

• Add optional prompt.island_system_messages with literal text or template names for each island. Missing and null entries use prompt.system_message.
• Choose the override from the child's sampling_island, including when its parent came from the archive.
• Validate the list length against database.num_islands, document the setting, and cover parallel execution and checkpoint resume.

Closes #293

Tests

• Focused tests: 51 passed.
• Full python -m unittest discover tests on Windows: 572 run, 1 failure, 3 errors. The remaining cases are existing Windows-specific failures in test_template_dir_resolution, test_api_key_from_env, test_reasoning_effort_config, and test_process_parallel (POSIX path expectation, temporary file locks, and process lookup semantics).
• Black, isort, and git diff --check passed.