Exclude Singapore and US West from deployable regions

[Che-Zhu]

Summary

Prevent the skill from offering or deploying to the Singapore and US West regions.

The current deploy configuration still uses https://usw-1.sealos.io as both the default region and an available region, so the exclusion must cover configuration, workflow behavior, examples, and tests rather than only hiding a menu option.

Required behavior

• Remove Singapore and US West region endpoints from the selectable deployment region list.
• Replace the current US West default with a supported region.
• Reject or skip excluded regions when they come from saved authentication state, custom input, dynamic discovery, or another fallback source.
• If an existing user configuration points to an excluded region, explain that it is unavailable and require selection of a supported region before deployment.
• Update all deployment-related examples, documentation, fixtures, evals, and tests that treat an excluded region as a valid target.
• Keep non-deployment test fixtures synthetic where a regional hostname is needed, without presenting excluded regions as supported choices.

Acceptance criteria

• Neither Singapore nor US West appears as a deployable or default region.
• The skill cannot start a new deployment against either excluded region through custom or cached input.
• Existing authentication for an excluded region is handled safely and produces an actionable re-selection flow.
• A supported replacement default is used consistently across configuration and user-facing guidance.
• Relevant validation and regression tests pass.