Feature Request: Statusline should show total context usage (matching /context)

Problem

The statusline shows token usage that differs from what /context displays, causing confusion about actual context consumption.

Example:

• /context shows: 43k/200k tokens (21%)
• Statusline shows: 0/200k

The discrepancy occurs because the statusline only tracks conversation messages, while /context shows total context including:

• System prompt (~4.6k tokens)
• System tools (~16.7k tokens)
• MCP tools (~18.2k tokens)
• Custom agents (~1.6k tokens)
• Memory files (~1.4k tokens)
• Messages

Why this matters

1. Trust: Users can't rely on the statusline to know their real context usage
2. Context management: The "fixed cost" of system prompt + tools is significant (~40k tokens) and invisible in the statusline
3. Workflow: Having to run /context frequently interrupts the flow

Proposed solution

Add an option to the statusline to display total context usage (matching /context), not just message tokens.

This could be:

• A new statusline variable like context_window.total_usage that includes system prompt, tools, MCP, agents, and memory
• Or a configuration option to toggle between "messages only" vs "total context" display

Current workaround

Run /context manually whenever accurate context usage is needed.