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

docs/SERVER-MODE.md

@@ -562,7 +562,7 @@ What runs as the member, and how:
   - In a member's folder, git and `gh` run as `sudo -n -u ah-<name> git …`, so git's own ownership check passes and every file stays the member's.
   - The editor and `.gitignore` saves go through `/opt/agenthub/libexec/write-as`.
   - A file uploaded in the Files tab (`POST /api/lanes/:id/git/upload`) is written the same way, AS the member through `write-as` (content on stdin), and only as a NEW file in an existing folder of the workspace — never over an existing name, never into `.git`, never a home dot folder, at most 25 MB. With **Download** on a file (`GET /api/lanes/:id/git/download`, read like any member file below, up to 512 MB) this is how a member with no SSH or `scp` to the host moves files in and out of their own folder; both work in a folder that is not a git repo.
-  - AgentHub's stored git logins are never handed to a member's git, and a password typed in the git bar goes into the member's OWN store (below) for that one call — through `write-as`'s stdin, never sudo's argv or env — and out again unless "remember" was ticked; a store that cannot be read is never rewritten (`409 ESTOREUNREADABLE`). The member keeps their OWN identity in Settings → Git → **Your git identity** (`/api/me/git`): a commit author (`user.name`/`user.email` in their `~/.gitconfig` — commits fail without one; a new member has none), SSH keys AgentHub makes for them (an ED25519 and/or an RSA one — AWS CodeCommit takes only RSA) and HTTPS logins, both under `~/.config/agenthub/` in their home, written as them by `write-as` (0600). Add workspace → **Clone** (`POST /api/lanes/clone`) clones as them with it and writes it into the clone's own config (`core.sshCommand` / `credential.helper=store --file=…`), so the git bar and their agents' pushes in that repo use it too. A plain `git` they type themselves in a pane reads none of that, so making the key also sets `core.sshCommand` in their `~/.gitconfig` (the key first, then `~/.ssh/id_*`; a switch under the key, issue #82) — unless they already set their own, which is left alone. A login typed in the Clone dialog goes to their store through `write-as`'s stdin, never sudo's argv or env, and is removed again after the clone unless "remember" was ticked.
+  - AgentHub's stored git logins are never handed to a member's git, and a password typed in the git bar goes into the member's OWN store (below) for that one call — through `write-as`'s stdin, never sudo's argv or env — and out again unless "remember" was ticked; a store that cannot be read is never rewritten (`409 ESTOREUNREADABLE`). The member keeps their OWN identity in Settings → Git → **Your git identity** (`/api/me/git`): a commit author (`user.name`/`user.email` in their `~/.gitconfig` — commits fail without one; a new member has none), SSH keys AgentHub makes for them (an ED25519 and/or an RSA one — AWS CodeCommit takes only RSA) and HTTPS logins, both under `~/.config/agenthub/` in their home, written as them by `write-as` (0600). Add workspace → **Clone** (`POST /api/lanes/clone`) clones as them with it and writes it into the clone's own config (`core.sshCommand` / `credential.helper=store --file=…`), so the git bar and their agents' pushes in that repo use it too. A plain `git` they type themselves in a pane reads none of that, so making the key also sets `core.sshCommand` in their `~/.gitconfig` (the key first, then `~/.ssh/id_*`; a switch under the key, issue #82) — unless they already set their own, which is left alone. The SSH keys are shown one section per kind of host, each with only its own steps: **GitHub · GitLab · Gitea · Bitbucket** (ED25519) and **AWS CodeCommit** (RSA: copy → upload in IAM → paste the SSH key ID `APKA…` and pick the region), plus a collapsed **Other SSH hosts (advanced)**. The key ID (or another host's user) is saved as `url."ssh://<user>@<host>/".insteadOf "ssh://<host>/"` in the same `~/.gitconfig`, so the plain `git clone ssh://git-codecommit.<region>.amazonaws.com/v1/repos/<repo>` works in a pane and in Clone. A login typed in the Clone dialog goes to their store through `write-as`'s stdin, never sudo's argv or env, and is removed again after the clone unless "remember" was ticked.
 - **Files a pane reads.** These are written into `~/.agenthub/handoff/` AS the member (`write-as`). The dashboard never writes, changes or deletes anything in a member's home itself: the member could have swapped the name for a link. The files are:
   - the workspace prompt and its context files;
   - the AI Proxy settings: no key, only that pane's own token for the gateway;