[Bug] Statusline JSON token counts don't match /context - possible calculation error

Bug Description

There appears to be a calculation discrepancy between the token counts provided in the statusline JSON current_usage fields and what the /context command reports. The statusline JSON is missing approximately 12k tokens (~6%) compared to /context.

Steps to Reproduce

1. Run /context command and note the percentage (e.g., 60%)
2. Check statusline which calculates from JSON fields: input_tokens + cache_read_input_tokens + cache_creation_input_tokens + output_tokens
3. Observe the discrepancy

Observed Behavior

| Source | Percentage | Tokens |
|--------|------------|--------|
| /context | 60% | 120k/200k |
| Statusline JSON | 54% | ~108k |
| Discrepancy | 6% | ~12k missing |

/context Breakdown

System prompt: 3.1k (1.6%)
System tools: 18.3k (9.2%)
MCP tools: 16.5k (8.2%)
Custom agents: 475 (0.2%)
Memory files: 10.3k (5.2%)
Messages: 25.8k (12.9%)
Autocompact buffer: 45.0k (22.5%)

The /context also lists Skills and slash commands separately (~18-20k tokens total), but these don't appear in the main percentage breakdown. It's unclear if/how these are being counted.

Expected Behavior

The statusline JSON current_usage fields should sum to the same total as /context reports (excluding the autocompact buffer which we add separately).

Questions for Investigation

1. Are Skills/slash command definitions included in current_usage?
2. Is there a calculation difference between how /context and statusline JSON count tokens?
3. Which components are included/excluded from each?

Environment

• OS: macOS Darwin 25.2.0
• Model: claude-opus-4-5-20251101