[Cowork] Allow clearing stale entries from a project's Outputs panel

### Preflight Checklist

- [x] I have searched [existing requests](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20label%3Aenhancement) and this feature hasn't been requested yet
- [x] This is a single feature request (not multiple features)

### Problem Statement

A Cowork project's Outputs panel accumulates a card for every file
delivered during a task, and there is no way to remove any of them.
No right-click menu, no hover affordance, no bulk clear.

This compounds with session lifecycle. Sessions run in ephemeral
containers that are reclaimed after inactivity, so hovering an older
card reveals a path to a file that no longer exists. Over time the
panel fills with dead references that cannot be dismissed, and the
useful entries get buried among them.

The panel also gives no indication of which entries are still
retrievable, so the only way to find out is to click each one.

### Proposed Solution

A remove action on each Output card, matching the Delete option that
artifacts already have in their ⋮ menu.

Additionally: mark cards whose underlying file is no longer
retrievable, so stale entries are identifiable without clicking, and
offer a "clear unavailable" bulk action.

### Alternative Solutions

- Auto-expiring Output cards when their session container is reclaimed.
Simpler, but destroys the record of what was produced, which has
value even when the file is gone.
- Persisting output files beyond the session so the paths stay valid.
Solves a different problem at much higher storage cost.
- Deleting and recreating the project. Not viable — it discards
instructions, context and memory to clear a list.

### Priority

Low - Nice to have

### Feature Category

Other

### Use Case Example

I run a Cowork project for technical writing whose
source files live in a git repo on my desktop. Over several sessions
Claude produced working copies of repo files — a draft audit, a spec
list, a FAQ doc — each of which was delivered into chat
and now appears as a card in the project's Outputs panel.

Two problems in daily use:

1. Most of those cards now point at files in sessions that have since
been reclaimed. The paths are dead. I can't tell which are live
without clicking each one, and I can't remove the dead ones.

2. The card names match filenames in my repo, so the panel reads like
a view of my project files when it isn't — those are point-in-time
copies, and the repo has moved on. A stale card is worse than no
card, because I might open one and work from an outdated draft.

What I want is to clear out the entries I no longer need, keeping the
panel as a short list of things I actually still use, so it stays
scannable and I'm not at risk of editing a stale copy.

### Additional Context

Claude desktop app 1.49585.0, Windows 11 (win32 x64).
Same account also used on macOS and iOS; the panel behaves the same
everywhere, so this is not platform-specific.

Related: #40043 requests a remove option for connected folders in a
project's context — the same missing-affordance pattern in a
neighboring panel.

Start by locating the Cowork project's Outputs panel and the existing artifact Delete option referenced in the issue. Trace how cards determine whether their delivered files are still retrievable, then verify the remove action, unavailable-state marking, and "clear unavailable" action against the stale-entry use case. Done means users can remove individual cards and bulk-clear unavailable ones without deleting the project.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.