/usage help text does not explain mixed metric scopes

找到 /usage 帮助文本和 issue 中引用的文档，然后检查命令输出如何标记其各个部分。更新措辞，明确本地活动、当前会话指标和计划周期配额的范围；验证帮助文本和渲染后的输出不再将所有指标描述为会话使用量。

Describe the bug

The /usage command help text says:

Display session usage metrics and statistics

However, the command output combines metrics with three different scopes:

1. Activity · last 180 days · N messages is aggregated from local history across multiple saved CLI sessions.
2. Changes, AI Credits, Tokens, and the per-model breakdown describe the current session.
3. Plan describes account/plan usage for the current quota period.

Calling all of this "session usage" is misleading. In a new session with only one user prompt, /usage can report hundreds of messages and show a six-month activity graph, making it appear that either the session boundary or message count is incorrect.

Affected version

GitHub Copilot CLI 1.0.81-11

Steps to reproduce the behavior

1. Use Copilot CLI across multiple sessions.
2. Start a new session and send one prompt.
3. Run /usage.
4. Observe that the help describes session metrics, while the output includes a 180-day, multi-session message count and chart plus plan-period quota usage.

Representative output:

Activity · last 180 days · 861 messages
...
Changes    +0 -0
AI Credits 2.72 (22h 1m 8s)
Tokens     ↑ 23.3k (11.8k cached) • ↓ 179
  gpt-5.6-sol ↑ 23.3k (11.8k cached) • ↓ 179
Plan       ... 0% used • resets in 27 days
           200 / 200,000 AIC

Expected behavior

The help text and documentation should make the scopes explicit. For example:

Display local activity history, current-session usage metrics, and plan quota usage

Ideally, the rendered output would also label each section's scope (for example, Local activity, Current session, and Plan period) so users do not have to infer it.

Additional context

• Operating system: Windows
• CPU architecture: x86_64
• Shell: PowerShell
• The current documentation describes /usage as showing "session statistics," including current-session AI Credits, duration, code edits, and token usage. It does not explain that the command also includes local multi-session activity and account/plan-period quota information.