feat(git): a CodeCommit SSH key ID is saved for every region; a clone tries each saved account

Settings → Git → AWS CodeCommit asks for the key ID only: the default key's
login is written for every CodeCommit region at once (allRegions), shown as
one row, with Use in every region for an older one-region save. A clone of
a CodeCommit URL with no user tries every saved CodeCommit login — the
region's own first, then other regions' and each named key's — so a second
AWS account clones from the plain AWS URL with no ssh://<alias>/ rewrite.

docs/API.md

@@ -1203,7 +1203,7 @@ GET    /api/me/git                 { account: "member"|"host", writable, ssh: {
 POST   /api/me/git/ssh-key         Make an SSH key — Body: { replace?, type?: "ed25519" | "rsa", name? } — without `name` the DEFAULT key of that type (`id_ed25519`/`id_rsa`), with `name` (lowercase letters, digits, `-`, ≤ 32; 400 EBADKEYNAME) one more beside it, `id_<type>_<name>` — for another AWS account or a second GitHub user. 409 EEXISTS when that key exists without replace, or the name is taken by a key of the other type; 409 ETOOMANYKEYS past 10 keys → 201, same shape
 DELETE /api/me/git/ssh-key         Delete one key — `?id=<key id>` (400 EBADKEYNAME for a malformed id), or the default key of `?type=ed25519|rsa` — or every key without either (404 ENOTFOUND, 400 EBADKEYTYPE). A login naming a deleted key goes back to every key; our terminal `core.sshCommand` follows the keys and goes with the last one
 PUT    /api/me/git/ssh-terminal    Use the keys for git typed in the account's own terminal — Body: { on } — sets/unsets `core.sshCommand` in its `~/.gitconfig` (every AgentHub key first, then `~/.ssh/id_*`; rewritten as keys are made or deleted while on); 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        An SSH login — Body: { host, user, alias?, key? } — writes `url."ssh://<user>@<host>/".insteadOf "ssh://<from>/"` to the account's `~/.gitconfig`, `<from>` = `alias` or the host itself (replacing ours for that host/alias), so `ssh://<from>/…` logs in as `user` (AWS CodeCommit: the IAM SSH key ID `APKA…`). `alias` (lowercase letters, digits, `-`, NO dot — 400 EBADALIAS) is one more account on the same host: two AWS accounts clone as `ssh://aws-prod/v1/repos/x` and `ssh://aws-dev/v1/repos/x`. `key` (a key id, 400 ENOKEY) is stored as `agenthub.<from>.sshKey` there: a clone of a `ssh://<from>/…` URL then offers ONLY that key (`IdentitiesOnly`) and persists it into the clone's config; without it every key is offered. An IAM SSH key ID logs in to every region of its AWS account, so a Clone of `ssh://git-codecommit.<region>.amazonaws.com/…` whose region has no login of its own borrows the ones saved for other CodeCommit regions (each with its own key): the first the remote accepts (`git ls-remote`), or the only one, and the clone's `origin` then names that user. 400 EBADHOST / EBADSSHUSER
+PUT    /api/me/git/ssh-user        An SSH login — Body: { host, user, alias?, key?, allRegions? } — `allRegions: true` (a CodeCommit host, no alias — else 400 EBADHOST) writes the rule for EVERY CodeCommit region at once (the list is `codecommitRegions` in `GET /api/me/git`), replacing each region's own; `DELETE /api/me/git/ssh-user/<a CodeCommit host>?allRegions=1&user=<key ID>` removes them all. Otherwise it writes `url."ssh://<user>@<host>/".insteadOf "ssh://<from>/"` to the account's `~/.gitconfig`, `<from>` = `alias` or the host itself (replacing ours for that host/alias), so `ssh://<from>/…` logs in as `user` (AWS CodeCommit: the IAM SSH key ID `APKA…`). `alias` (lowercase letters, digits, `-`, NO dot — 400 EBADALIAS) is one more account on the same host: two AWS accounts clone as `ssh://aws-prod/v1/repos/x` and `ssh://aws-dev/v1/repos/x`. `key` (a key id, 400 ENOKEY) is stored as `agenthub.<from>.sshKey` there: a clone of a `ssh://<from>/…` URL then offers ONLY that key (`IdentitiesOnly`) and persists it into the clone's config; without it every key is offered. An IAM SSH key ID logs in to every region of its AWS account, so a Clone of `ssh://git-codecommit.<region>.amazonaws.com/…` tries every CodeCommit login saved — the region's own first, then other regions' and every named key's (another AWS account), each with its own key: the first the remote accepts (`git ls-remote`), or the only one, and the clone's `origin` then names that user. So a second account's key clones from a plain AWS URL too, with no `ssh://<alias>/` rewrite. 400 EBADHOST / EBADSSHUSER
 DELETE /api/me/git/ssh-user/:from  Drop the rule of that host or alias and its key link (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)