feat(docker): opt-in gh + az CLIs with git credential helpers for private repos

Add Case -> Clone Repo could only reach public repositories in the Docker
deployment. This lets a deployment opt in to the GitHub CLI and the Azure
CLI (+ azure-devops extension) as git credential helpers. Codeman itself
still collects no credentials.

- server.Dockerfile / agent.Dockerfile: CODEMAN_INSTALL_GH /
  CODEMAN_INSTALL_AZ build args (0 or 1, default 0; anything else stops the
  build). Off leaves no apt repository, package, extension, helper script
  or credential entry, so a default build is unchanged. On installs from
  the vendors' apt repositories and configures system gitconfig helpers:
  github.com / gist.github.com -> `gh auth git-credential`, dev.azure.com /
  *.visualstudio.com -> new docker/git-credential-azure-cli (an Entra ID
  token from `az account get-access-token`, or AZURE_DEVOPS_EXT_PAT).
  A helper whose CLI is not signed in prints nothing, so a private clone
  still fails fast.
- The extension lives in AZURE_EXTENSION_DIR outside HOME
  (/opt/codeman-az-extensions, runtime-owned; /opt/az-extensions, gid-0
  group-writable in the agent image).
- Hosts turn them on in docker-compose.override.yml: `build: args:` for the
  server image, `environment:` CODEMAN_AGENT_IMAGE_INSTALL_GH / _AZ for the
  agent image. build-agent-image.mjs and the in-app auto-build share one
  env -> ARG table (pinned by the parity test) and pass nothing when unset.
  docker-compose.yaml is untouched; .env.example only gains a comment, so
  the self-updater's environment gate sees no new keys.
- Docker cases seed the gh sign-in (~/.config/gh/hosts.yml, config.yml) and
  the az sign-in files from ~/.azure per file, read-only, like pi/grok.
- The Clone Repo AUTH_REQUIRED message says how to sign the server's git
  in instead of claiming private repositories cannot be cloned.
- Docs: docker/README.md "Private repositories", docker-compose.md,
  docker-cases.md, the Quick-Start / Core-Concepts / Docker-Cases wiki
  pages, security-architecture.md, architecture-invariants.md, changeset.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0167CiuzLrmjYWxwKp3rMWjw

docs/wiki/Core-Concepts.md

@@ -20,12 +20,19 @@ Three ways to get one, all under **+** next to the case picker:
 | How               | Result                                                                                                 |
 | ----------------- | ------------------------------------------------------------------------------------------------------ |
 | **Create New**    | A fresh `~/codeman-cases/<name>` with a scaffolded `CLAUDE.md`.                                          |
-| **Clone Repo**    | A public repo cloned into `~/codeman-cases/<name>` and registered as a case.                            |
+| **Clone Repo**    | A repo cloned into `~/codeman-cases/<name>` and registered as a case. Private repos need this machine's own git credentials (see below). |
 | **Link Existing** | An existing folder anywhere on disk, registered in place. Nothing is copied or moved.                    |
 
 Linked cases keep living where they are. Deleting a case in Codeman removes the
 registration, and for a linked case that is all it removes.
 
+**Clone Repo never asks for credentials.** It uses whatever the server's own git already has:
+an ssh key, or a credential helper such as `gh auth setup-git`. The Docker image can include
+helpers for GitHub (`gh`) and Azure DevOps (`az`), turned on in `docker-compose.override.yml`;
+then signing those CLIs in once from a shell session is enough. See the private repositories
+section of `docker/README.md`. Without credentials a private repo fails straight away with an
+authentication error.
+
 **Cases created from scratch are the only copy of that code.** Uninstalling Codeman does not
 delete `~/codeman-cases/`, but treat that directory as real work, not scratch space.