[FEATURE]: Show cached token count inline in TUI context panels

[bainos]

• I have verified this feature I'm about to request hasn't been suggested before.
  When a provider returns cache token usage the TUI shows no indication of it. Displaying cached tokens inline (e.g. 16,570 tokens (16,534 cached)) gives visibility into whether caching is active. Only shown when cached > 0, no visual change otherwise.
  Related to [FEATURE]: Display token usage information in the TUI #13003 (narrower scope — cached count only, no new UI elements).

[github-actions]

This issue doesn't fully meet our contributing guidelines.

What needs to be fixed:

• The feature request template was not used. The required verification checkbox ("I have verified this feature I'm about to request hasn't been suggested before") is missing.

Please edit this issue to address the above within 2 hours, or it will be automatically closed.

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This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: Display token usage information in the TUI #13003: Feature request to display token usage information in the TUI — explicitly includes showing cached tokens (e.g. in a /usage command breakdown)

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