[Feature Request] Add custom session tags for cost attribution in the Copilot consumption overview

### Feature summary

Allow sessions to be tagged with custom labels (e.g. project, ticket, feature) so that those tags appear as a dimension in the Copilot consumption / cost breakdown.

### What problem are you trying to solve?

Our team works on many projects, features, and tickets in parallel. The consumption overview already breaks down cost by model, user, and day, which is useful, **but none of those dimensions map to what we were actually working on.**

**Today we can't tell which ticket or feature was cheap and which was expensive, so we can't attribute Copilot spend back to the work that drove it**. This matters for internal cost reporting and for deciding where Copilot usage pays off. Maybe this can even be used to invoice spent tokens to our customers.

### Proposed solution

Add the ability to attach one or more custom tags to a session, surfaced as a filter/grouping in the existing consumption overview.
Specifically:

- **Manual tagging per session**: set a tag (or tags) when starting or during a session, e.g. project-A, ticket#1234, feature-xyz.
- **Project-level default tag**: a project setting where a tag like project-A is automatically applied to every session in that project, so per-ticket tagging is the only manual step.
- **Multiple tags per session**: a single session can carry several tags at once (e.g. project-A + ticket#1234), since one piece of work usually belongs to more than one category.

**Where it should appear**: In the existing consumption overview, as an additional breakdown/filter dimension alongside model, user, and day.

Example: A developer working on ticket#1234 in Project A would have sessions automatically tagged project-A (project default) and could add ticket#1234 manually. The consumption overview would then let us filter or group by tag to see total tokens/cost for that ticket or for Project A as a whole.

**Why it matters**: This turns the consumption data into something teams can act on. Attributing spend to real work, comparing cost across tickets/features, and justifying or right-sizing Copilot usage per project.

This is probably something that the underlying Copilot SDK can't do right now, but I hope that is as easy as "adding attributes to a session" 😅 Even if the cost breakdown is not possible, adding tags would be a necessary first step.

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

_No response_

まず、既存の Copilot の利用状況概要と、メタデータを保持できそうなセッションモデルまたはエントリポイントを特定します。基盤となる Copilot SDK がセッション属性をサポートしているか確認し、モデル、ユーザー、日というディメンションが概要にどのように到達するかを追跡します。完了条件は、セッションが1つ以上の手動タグまたはプロジェクトのデフォルトタグを持てること、そして概要でそれらによってコストをフィルタリングまたはグループ化できることです。SDK の制限は解決するか、文書化します。

索引モデルが issue の本文から書いたものです。