providers: add remove command for saved provider profiles

[Ashwinhegde19]

Problem

zero providers add lets users save provider profiles, and zero providers use switches between them, but there is currently no CLI command to remove a saved provider profile when it is no longer needed. Users have to manually edit ~/.config/zero/config.json, which is easy to get wrong and is less discoverable than the existing provider management commands.

This becomes noticeable when trying temporary providers, switching between OpenCode/Kilo/OpenAI-compatible endpoints, or cleaning up old local endpoints.

Proposed scope

Add a focused provider removal command, for example:

zero providers remove <name>

Expected behavior:

• remove the matching saved provider profile from config
• return a clear error when the provider name does not exist
• handle the active provider case explicitly
  • either clear activeProvider, or switch to the next remaining provider
  • maintainer preference welcome here
• support --json consistently with other provider commands if applicable
• add focused tests for existing provider, missing provider, and active-provider removal

Notes

This should only manage Zero provider profiles. It should not delete environment variables or external provider accounts. If stored API key cleanup is desired, that can be a separate follow-up depending on how maintainers want secrets handled.

I can work on this if maintainers agree with the scope. Please add issue-approved if this is acceptable for a community PR.

[Vasanthdev2004]

Closing as completed — the provider manager work (#560) added a zero providers remove <name> command (see zero providers remove <name> in the command center, backed by the saved-profile removal path in the provider manager), so saved provider profiles can now be removed from the CLI instead of hand-editing config.json. Reopen if the removal behavior doesn't match what was asked.