feat(tui): add session statistics display (cost, tokens, cache rate, context usage)

[JeremyDev87]

Purpose

Display real-time session statistics in the codingbuddy TUI status bar, similar to OMC's status line. Users should see cost, token usage, cache hit rate, and context window utilization at a glance without needing OMC installed.

Reference: OMC Status Line

[OMC#4.2.7] | session:173m | 🔴 | ~$0.6346 | 419.9k | Cache: 99.9% | $0.22/h | ctx:42%

Target Display

[CB] | session:173m | ~$0.63 | 419.9k tokens | Cache: 99.9% | $0.22/h | ctx:42%

Data Points

Metric	Description	Data Source
session	Session elapsed time	MCP server start timestamp
cost	Estimated cumulative cost	Token count × model pricing
tokens	Cumulative token usage	Track from MCP tool call context
cache	Prompt cache hit rate	cache_read vs cache_creation tokens
$/h	Cost per hour	cost ÷ session hours
ctx	Context window usage %	Current tokens ÷ model max context

Technical Investigation Needed

Data Collection Approaches

1. MCP server self-tracking — Count tool calls, estimate tokens from request/response sizes
2. Claude Code metadata API — Check if Claude Code exposes session stats via API or env vars
3. Anthropic API usage headers — x-ratelimit-* headers provide rate limit info
4. Hybrid — Combine MCP tracking with any available Claude Code metadata

TUI Implementation

• Existing TUI: apps/mcp-server/src/tui/ (Ink/React-based)
• Add status bar component at the bottom of TUI
• Update interval: every 30 seconds or on each tool call

Changes

• apps/mcp-server/src/tui/ — New status bar component
• apps/mcp-server/src/mcp/ — Session statistics tracking service
• apps/mcp-server/src/ — Token/cost calculation utilities

Acceptance Criteria

• Session elapsed time displayed
• Estimated cost displayed (based on token counting)
• Token count tracked and displayed
• Cache hit rate calculated and displayed (if data available)
• Context usage percentage displayed (if data available)
• Cost per hour calculated
• Updates in real-time (or near real-time)
• Works without OMC installed
• TUI tests updated

References

• OMC status line implementation (inspiration, not copy)
• codingbuddy TUI: apps/mcp-server/src/tui/
• Anthropic API pricing: per-model token costs
• Claude Code API: check for session metadata exposure

[JeremyDev87]

Superseded by #825 (plugin real-time session statistics). Plugin can access API metadata directly for exact stats, making MCP-based estimation unnecessary.