feat(git identity): one section per git host; CodeCommit SSH key ID makes a plain ssh:// clone work

AWS CodeCommit logs an SSH key in by its IAM SSH key ID (APKA…), so a
plain `git clone ssh://git-codecommit.<region>.amazonaws.com/v1/repos/x`
was refused as the Linux account name. The account can now save an SSH
user per host — PUT/DELETE /api/me/git/ssh-user, listed as `sshUsers` —
written as `url."ssh://<user>@<host>/".insteadOf "ssh://<host>/"` in its
own ~/.gitconfig (as the member under S3), so the plain URL works in a
pane, in Clone and in the git bar. Only rules of exactly that shape are
listed or removed; any other insteadOf is the user's and left alone.

Settings → Git → Your git identity now shows the SSH keys one section
per kind of host, each with only its own numbered steps: GitHub · GitLab
· Gitea · Bitbucket (ED25519) and AWS CodeCommit (RSA: copy → upload in
IAM → paste the key ID and pick the region, then a "Ready" clone line),
plus a collapsed "Other SSH hosts (advanced)". The shared key-type
picker is gone: each section has its own Generate/Replace/Delete.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/API.md

@@ -1046,6 +1046,8 @@ GET    /api/me/git                 { account: "member"|"host", writable, ssh: {
 POST   /api/me/git/ssh-key         Make the SSH key of `type` — Body: { replace?, type?: "ed25519" | "rsa" } (409 EEXISTS when that TYPE exists, without replace; the other type is kept — an account may hold one of each, and every ssh git offers both) → 201, same shape
 DELETE /api/me/git/ssh-key         Delete the key of `?type=ed25519|rsa`, or every key without it (404 ENOTFOUND, 400 EBADKEYTYPE) — also removes our terminal `core.sshCommand`
 PUT    /api/me/git/ssh-terminal    Use the key for git typed in the account's own terminal — Body: { on } — sets/unsets `core.sshCommand` in its `~/.gitconfig` (ours first, then `~/.ssh/id_*`); on by default when a member makes a key; 409 EFOREIGNSSH over one the user wrote, which is never touched
+PUT    /api/me/git/ssh-user        The SSH login name for one host — Body: { host, user } — writes `url."ssh://<user>@<host>/".insteadOf "ssh://<host>/"` to the account's `~/.gitconfig` (replacing ours for that host), so a plain `ssh://<host>/…` URL logs in as `user` (AWS CodeCommit: the IAM SSH key ID `APKA…`); 400 EBADHOST / EBADSSHUSER. Listed as `sshUsers: [{host, user}]` in GET /api/me/git
+DELETE /api/me/git/ssh-user/:host  Drop that rule (404 ENOTFOUND); any other `insteadOf` the user wrote is never touched
 POST   /api/me/git/https           Save an https login — Body: { host, username, password }
 DELETE /api/me/git/https/:host     Delete one (404 ENOTFOUND)
 PUT    /api/me/git/author          Commit author — Body: { name, email } (400 EBADAUTHOR) — `git config --global user.name/email` as the account