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>

server/routes/git-identity.js

@@ -3,7 +3,8 @@
  * mounted at `/api/me/git`: the SSH keys a clone uses — ed25519 and/or rsa (make, replace, delete —
  * only the PUBLIC key is ever returned) and the HTTPS logins (save, delete —
  * `hasToken` only, never a password), whether the account's own terminal
- * git uses that key (`core.sshCommand`), and the commit author (`user.name` /
+ * git uses that key (`core.sshCommand`), the SSH login name per host
+ * (`url.<…>.insteadOf` — AWS CodeCommit's SSH key ID), and the commit author (`user.name` /
  * `user.email` of the account's global git config — a member's commits fail
  * without one). The account is the signed-in user's
  * own Linux account under S3, else the host's, which only an admin (or local
@@ -29,6 +30,7 @@ const STATUS = {
   ESTOREUNREADABLE: 409,
   EBADKEYTYPE: 400,
   EFOREIGNSSH: 409,
+  EBADSSHUSER: 400,
 };
 
 function fail(res, e) {
@@ -47,6 +49,7 @@ async function view(account) {
     ssh: keys[0] ? { ...keys[0], terminal } : null,
     sshKeys: keys,
     sshTerminal: terminal ?? null,
+    sshUsers: await identity.listSshUsers(account),
     https: identity.listHttps(account),
     author: await identity.getAuthor(account),
   };
@@ -115,6 +118,32 @@ router.put("/ssh-terminal", sameOriginGuard, async (req, res) => {
   }
 });
 
+/**
+ * The SSH user name for one host (`{host, user}`) — e.g. AWS CodeCommit's SSH
+ * key ID, so a plain `ssh://git-codecommit…/` URL logs in. DELETE drops it.
+ */
+router.put("/ssh-user", sameOriginGuard, async (req, res) => {
+  try {
+    const account = accountOf(req, { write: true });
+    await identity.setSshUser(account, req.body || {});
+    res.json(await view(account));
+  } catch (e) {
+    fail(res, e);
+  }
+});
+
+router.delete("/ssh-user/:host", sameOriginGuard, async (req, res) => {
+  try {
+    const account = accountOf(req, { write: true });
+    if (!(await identity.removeSshUser(account, req.params.host))) {
+      return res.status(404).json({ error: { code: "ENOTFOUND", message: "no such SSH user" } });
+    }
+    res.json(await view(account));
+  } catch (e) {
+    fail(res, e);
+  }
+});
+
 router.post("/https", sameOriginGuard, async (req, res) => {
   try {
     const account = accountOf(req, { write: true });