[FEATURE] Persistent status line showing context usage, cost, and model in CLI

Problem

During a Claude Code CLI session, there is no persistent indicator of:

• Context window usage (% filled, tokens used/remaining)
• Session cost (cumulative $ spent)
• Current model being used

The only way to check is /cost and /status, which interrupts the workflow. In long sessions (especially with large file reads, MCP tools, and conversation compaction), knowing how close you are to context limits is critical for deciding when to start a new session or compress context.

Proposed Solution

Add a persistent status line (bottom bar) in the CLI TUI showing:

[claude-opus-4-6] Context: 45% (180k/400k) | Cost: $12.34 | Duration: 1h 23m

Key elements:

• Model name — which model is currently active
• Context window — percentage and token count (used/max)
• Session cost — cumulative spend
• Optional: duration — wall time

Configuration

Allow users to toggle and customize via settings.json:

{
  "statusLine": {
    "enabled": true,
    "show": ["model", "context", "cost"]
  }
}

Why This Matters

• Long sessions with many tool calls can silently approach context limits
• Context compaction happens without warning — a status bar would make it predictable
• Cost awareness helps users make informed decisions about model switching (related: #19269)
• Power users running expensive operations (web search, large file reads, MCP calls) need real-time feedback

Current Workarounds

• /cost — shows cumulative cost but interrupts workflow
• /status — shows session info but no context usage
• No way to see context window fill level at all

Environment

• Claude Code CLI (terminal, not VS Code)
• macOS / Linux / WSL