fix(agents): retain original agent keys in remapAgentKeysToDisplayNames to prevent crash

[code-yeongyu]

Problem

remapAgentKeysToDisplayNames drops original agent keys, causing crashes when agents are accessed by original keys.

Root Cause

The function replaces original keys with display names but doesn't retain the original key mappings.

Fix

Keep both original key and display name key in the agents map.

Validation

• bun test src/shared/ --timeout 30000
• bun run typecheck
• bun run build

Closes #1922

---

Summary by cubic

Retain original agent keys when remapping to display names to prevent crashes when agents are accessed by their original keys. Resolves Linear #1922.

• Bug Fixes
  • Keep both original and display name keys in the agents map.
  • Update tests to confirm original keys remain and reference the same agent as the display name.

^{Written for commit df02c73. Summary will update on new commits.}

[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: The change is purely additive, ensuring original keys remain accessible alongside display names. Tests confirm correct behavior and no regressions in existing lookups.}