feat(opencode): add /context usage dialog

[bpeers01]

Summary

• Add a /context command that surfaces last-request token usage and limits in the session dialog.
• Show cache hit breakdown and a fallback when limits are unavailable.

Verification

• Open a session, type /context, confirm dialog opens and shows usage bars.
• With no prior model request, confirm the “No model requests yet” message.
• With a model missing limits, confirm the “Model limits unavailable…” message.

Screenshots

• Autocomplete for /context (CLI command)
  
• Context usage dialog (bars)
  
• Before: N/A (new command)

TUI-only UI change; happy to go through design review if needed.

Fixes#10575

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found several potentially related PRs:

Related PRs:

1. feat: add /stats dialog with token usage statistics - Very similar, adds a dialog to display token usage statistics, likely addressing the same or overlapping feature space.

2. feat(tui): add /usage command and sidebar usage section - Adds a /usage command to display usage information in TUI, similar functionality to the /context command.

3. feat: add usage tracking API and /usage dialog - Earlier implementation of usage dialog functionality.

4. feat(tui): add context preview dialog - Older PR that added a context preview dialog, may have overlapping concerns.

These PRs may be addressing the same feature area of token usage and context information display in the TUI. PR #10234 (stats dialog) appears to be the most likely duplicate or closely related PR since both target token usage statistics display.

[bpeers01]

Heads up: this overlaps with #6152 conceptually, but this PR is intentionally smaller. It only surfaces existing last-request token metrics via a /context dialog, with no new endpoint and no token estimation.

[wenkangjing]

Nice one, would love to see this merged

[github-actions]

Closing this pull request because it has had no updates for more than 60 days. If you plan to continue working on it, feel free to reopen or open a new PR.