fix(agent): preserve side chat workspace on migration

[cjvayhxy-dev]

Summary

• Preserve the owner workspace when an Agent history or file-preview side chat migrates to Agent mode.
• Use one workspace-resolution rule in both migration entry points, with valid current-workspace fallback.
• Hide the global Agent/Chat mode switcher from fixed Chat side panels.

Validation

• Added workspace-resolution and side-chat empty-state regression tests (7 passing assertions).
• Ran Electron typecheck, renderer build, diff check, and manual UI verification.
• Completed an independent code review with no blocking findings.

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