feat(agents): add opencode provider merge mode

[Golosay]

Summary

Add --merge-providers to codemie-opencode so the launcher can merge
CodeMie's generated OpenCode provider config with the user's existing
OpenCode setup instead of replacing it outright.

This also removes the injected enabled_providers whitelist in merge
mode, which lets OpenCode keep showing authenticated native providers
such as corporate GitHub Copilot alongside CodeMie SSO models.

Changes

• add the --merge-providers flag to codemie-opencode
• merge CodeMie-owned OpenCode providers with the user's config without
  duplicating providers or stale CodeMie model lists
• preserve CodeMie proxy and SSO behavior by keeping codemie-proxy
  authoritative while still allowing other providers to remain visible
• remove the merge-mode provider whitelist so authenticated OpenCode
  providers like corporate Copilot are not filtered out
• add unit and integration coverage for merge behavior and whitelist
  removal
• document the new launch mode in the README

Impact

Before this change, launching with codemie-opencode replaced the
visible OpenCode providers with a CodeMie-only set. That prevented users
from switching to already-authenticated corporate Copilot models inside
OpenCode.

With codemie-opencode --merge-providers, users can launch OpenCode with
CodeMie SSO models and corporate Copilot models in the same session.
This improves usage management because teams can keep CodeMie-governed
model access available while still using approved corporate Copilot
capacity when it makes sense.

Checklist

• Self-reviewed
• Manual testing performed
• Documentation updated (if needed)
• No breaking changes (or clearly documented)

[codemie-ai]

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[Golosay]

Hello colleagues! Any decision with this PR?)