feat(tui): context inventory — list and remove attached/prompt files (`/drop`)

[aheritier]

Summary

Let users see what files are currently in context and remove them — an Aider-style /add / /drop inventory. docker-agent has /attach and @file references but no way to view or remove them.

Proposed behavior

• Extend the /context view (or add a /drop command) to list attached files and prompt files, each with a per-item token estimate.
• Allow removing an attachment from the session.

Implementation notes (starting points)

• Add a removal API to Session.AttachedFiles in pkg/session/session.go (~718-745)
• pkg/tui/dialog/context.go, pkg/tui/commands/commands.go
• Keep @ref completion consistent in pkg/tui/components/editor/editor.go

Complexity: M. (Note: this pairs naturally with the /context view but should be shippable on its own against the current attachment model.)

Background: shared plan context-management-gap-analysis (Phase 2, item 4).

Dependencies / Relationships

• Blocked by: feat(tui): add /context command with a categorized context-window breakdown #3432 (extends the /context dialog)

[aheritier]

take care @Sayt-0 that this one is supposed to extend /context to build in #3432 cc @dgageot

[Sayt-0]

Yes, I realized that afterwards. I'll fix that late