[FEATURE]: Display token usage information in the TUI

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Problem

Currently, token usage information (input tokens, output tokens, remaining budget) is tracked internally but not visible to users in the OpenCode TUI. This makes it difficult for users to:

• Monitor costs in real-time during conversations
• Understand context window consumption
• Track usage against budget limits
• Make informed decisions about when to compact sessions

Proposed Solution

Display token usage information in the OpenCode TUI interface. Possible implementation approaches:

Option 1: Status Bar Display
Add token usage to the bottom status bar (similar to how file path or mode is displayed):

┌─────────────────────────────────────────────────┐
│ [Build Mode]  Tokens: 19.9K/1M (980K remaining) │
└─────────────────────────────────────────────────┘

Option 2: New /usage or /tokens Command
Add a slash command that displays detailed token usage:

/usage

Current Session:
  Input tokens:     12,450
  Output tokens:     7,477
  Cached tokens:     5,200
  Total:            19,927 / 1,000,000
  Remaining:       980,073 (98%)
  
Estimated cost: $0.05 (using claude-sonnet-4-5)

Option 3: Toggle-able Detail View
Add a keybind (e.g., ctrl+x t for tokens) to toggle token usage display in the conversation view, showing per-message token counts.

Option 4: Combination Approach

• Lightweight display in status bar for at-a-glance monitoring
• Detailed /usage command for comprehensive breakdown
• Optional per-message display via toggle

Benefits

1. Cost Awareness: Users can monitor spending in real-time, especially important for pay-as-you-go models
2. Budget Management: Helps users stay within monthly limits (especially relevant for OpenCode Zen users with spending caps)
3. Context Window Management: Users can see when they're approaching token limits and need to compact
4. Transparency: Makes the AI interaction more transparent and understandable
5. Educational: Helps users learn which operations consume more tokens

Additional Context

The token usage data is already being tracked (visible in system messages on the backend), so this feature would primarily involve exposing existing data to the UI rather than implementing new tracking logic.

Related Features

• Ties into the existing /compact command by helping users understand when compaction is needed
• Complements OpenCode Zen's monthly usage limits feature
• Could integrate with the auto-compaction feature (compaction.auto config)

Suggested Configuration

Allow users to customize this via config:

{
  "tui": {
    "show_token_usage": true,
    "token_display_format": "compact"
  }
}