fix(messages): restore main typecheck for pasted mentions

[loganj]

🤖

Summary

• Main does not build. Two pull requests merged within minutes of each other and each was correct alone: feat(messages): copy and paste mentions, channel refs and links #579 (paste mentions) checked pasted identity links with a helper that refactor(mentions): let the Mentions plugin own what the chooser offers #669 (Mentions plugin owns the chooser) moved out of the app shell. TypeScript now fails in MessageComposer.tsx.
• This change restores the build without changing what users see. When you paste a link to a person, the composer still notifies them under their current profile name, never the pasted label. A key with no known name stays plain text unless that person is in the channel. Edits still never add recipients.

Details

• New pastedMentionRecipient in mention-admission.ts, the app shell's rule for who a draft may address. It uses mentionAdmission, then names the recipient from the profile, the session roster, or the agent list.
• The app shell no longer depends on the Mentions plugin's chooser list, which keeps the boundary that refactor(mentions): let the Mentions plugin own what the chooser offers #669 set.
• feat(messages): copy and paste mentions, channel refs and links #579's composer paste tests pass unchanged. A new unit test covers named outsiders, unnamed members, agents, unknown keys and archived keys.