[txltedxgod]

Summary

Closes #144

Consolidates the three independent implementations of 'format template + write' onto \write_starter_config\ and ensures all starter-config writes are atomic:

1. Atomic starter creation: \write_starter_config\ now writes via \write_text_atomic\ (temp file in same directory + \os.replace), preventing corrupted/truncated .profiles\ files on interrupted --init\ or creation prompts.
2. Consolidated delegation:
   • \profiles.app.init_default_config\ delegates directly to \write_starter_config.
   • \profiles.core.config.loader.propose_config_creation\ delegates directly to \write_starter_config.
   • \profiles.launch.creation.propose_creation\ (and the /api/config/create\ endpoint) continues delegating to \write_starter_config.
3. Custom \cwd\ parameter: \write_starter_config\ now supports an optional \cwd\ keyword argument (defaulting to \Path.cwd()) for template placeholder substitution.
4. Test coverage:
   • Added \ est_uses_atomic_write\ and \ est_writes_with_custom_cwd\ to \ ests/core/test_actions.py.
   • Added \ est_delegates_to_write_starter_config\ to \ ests/test_app.py.
   • Updated \ est_write_error_returns_false\ in \ ests/core/config/test_loader.py\ to verify atomic writer error handling.

All 227 configuration and creation tests pass cleanly.