Clone Repositories into a Workspace with GitHub tokens over a unix socket

[ExtraToast]

Design: #59 (the glossary and ADRs). Use the glossary's terms.

What to build

The user creates a Repo-backed Workspace. Its primary Repository is cloned, and the user can Add Repository to clone more. Workspace Status moves Preparing to Ready, or to Failed with a reason the UI shows. Agents push and pull with short-lived GitHub App installation tokens scoped to the Workspace's Repositories. A git credential helper gets each token over a unix socket, and the API accepts only callers whose peer uid is the agent user. The shared bearer between runner and API is removed. agents-ui uses "Add Repository" wording.

Implements ADR 0003: The API and agents run as separate users.

Acceptance criteria

• Workspace Status is Preparing, Ready, Failed or Destroyed. Starting and Idle are no longer set.
• A Repo-backed Workspace clones its primary Repository. Add Repository clones another into the same Workspace.
• git push from an Agent Session succeeds with a token scoped to that Workspace's Repositories, and fails for any other repository.
• A socket request from a process running as api or root is rejected. A test covers the peer uid check.
• The GitHub App private key is never readable by agent.
• The shared token bearer secret is no longer read or required.

Blocked by

• Tracer bullet: run a Shell Agent Session in a Scratch Workspace inside the API container #62