[Feature] Add /status command to show session state and token usage

[Prodesire]

Problem or Motivation

There is no single command to view the current session state. Users need to run /model, /auth, and mentally track token consumption separately. Key information like cumulative token usage, active provider, default region, session ID, and working directory are scattered or unavailable.

This is especially useful for:

• Monitoring costs during long sessions
• Debugging provider/model configuration issues
• Quickly verifying which credentials and region are active before a deployment

Proposed Solution

Add a /status command that displays a concise overview of the current session:

╭─ Session Status ─────────────────────────────────────────────╮
│  Session:   e91c5730 (resumed)                               │
│  Provider:  DashScope (qwen3.7-max)                          │
│  Region:    cn-beijing                                       │
│  CWD:      /Users/dev/infra                                  │
│                                                              │
│  Token Usage (this session):                                 │
│    Input:   12,450 tokens                                    │
│    Output:   3,280 tokens                                    │
│    Cache:    8,200 tokens (read)                              │
│    Total:   15,730 tokens                                    │
│                                                              │
│  Turns:     7 / 100                                          │
│  Context:   45% used (58k / 128k)                            │
╰──────────────────────────────────────────────────────────────╯

Key fields:

• Session ID and whether it was resumed
• Provider + model currently active
• Default region for cloud operations
• Cumulative token usage (input, output, cache read/create) for the session
• Turn count vs max turns
• Context window utilization (current tokens / window size)

Related Area

CLI / Interactive Mode

Additional Context

The renderer already tracks per-message token usage (_format_token_count in renderer.py), and ContextManager.get_usage() provides context utilization data. The main work is accumulating session-level totals and wiring them into a new slash command.

[guima-why]

Implemented and merged in PR #65.

• Added the /status command for session state and token usage display.
• Usage reporting follows the product decision not to show cache creation tokens; Total is input + output.
• Usage sidecar storage is aligned with the new session directory format.

Closing as completed.