fix: retain original agent keys in remapAgentKeysToDisplayNames

[Wongbuer]

Summary

• Fixes remapAgentKeysToDisplayNames() dropping original agent keys, which causes upstream opencode's Agent.get() to return undefined → TypeError: undefined is not an object (evaluating 'agent.name')
• Both display name and original key are now preserved in the agent map

Fixes #1922

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Summary by cubic

Preserves original agent keys when remapping to display names so Agent.get() can look up by the original key without crashing. The agent map now contains both the display name and original key for each known agent.

• Bug Fixes
  • remapAgentKeysToDisplayNames now adds both display name and original key entries; unknown keys remain unchanged.
  • Updated tests to assert presence of both keys.
  • Fixes remapAgentKeysToDisplayNames drops original agent keys, causing Agent.get() to return undefined #1922.

^{Written for commit 9965549. Summary will update on new commits.}

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[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Auto-approved: Changes only add preservation of original keys alongside display names, no regression risk detected by review.}