Suppress "remove" button on auto-attached charms in chatbot.tsx

[bfollington]

Summary by cubic

Hide the remove (X) button for auto‑attached charms in the chat attachments bar, per Linear CT-1021. User-added attachments remain removable.

• Bug Fixes
  • Added a per-attachment removable flag (default removable unless set to false).
  • Auto-attached charm sets removable: false; user-added mentions set removable: true.
  • CTAttachmentsBar now hides the remove button when attachment.removable === false while still honoring the bar’s removable prop.

^{Written for commit f6cd907. Summary will update automatically on new commits.}

[linear]

CT-1021 Suppress (x) to remove attachment for auto-attached charm

[cubic-dev-ai]

No issues found across 2 files

[cubic-dev-ai]

No issues found across 1 file