fix(docker): gate gh/az seeding on its switch; no shared git sign-in for non-admin clones

Addresses the review on #472.

- CRED_STORES: `.config/gh` and `.azure` now carry `enabledByEnv`
  (CODEMAN_AGENT_IMAGE_INSTALL_GH / _AZ), and resolveDockerCredentialArtifacts
  skips a store unless that variable is exactly `1`, read at container
  create. A host that merely has ~/.config/gh/hosts.yml or a plaintext MSAL
  cache no longer copies them into every case container. Tests: the default
  environment seeds neither even with the files present, and each store
  follows only its own switch.
- Multi-user mode: a non-admin's Clone Repo clone and preflight run with
  `git -c credential.helper=` (GIT_NO_CREDENTIAL_HELPERS, placed before the
  subcommand), so the server account's helpers are never lent to them.
  Verified against a real private repo that it also clears the URL-scoped
  credential.<url>.helper entries, and that public clones still work.
  Tests: the argv in test/git-clone.test.ts, and the route decision
  (non-admin cleared; admin and single-user kept) in
  test/routes/case-clone-credential-helpers.test.ts.
- Docs: recreate the case container to pick up seeds (docker/README.md,
  Docker-Cases wiki, docker-cases.md); the multi-user behaviour in
  docker/README.md and security-architecture.md; "functionally unchanged"
  instead of "unchanged" for an image built with both switches off
  (server.Dockerfile comment, README, changeset).

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

test/git-clone.test.ts

@@ -201,6 +201,25 @@ describe('buildCloneArgs / buildLsRemoteArgs', () => {
       'd',
     ]);
   });
+
+  it('clears every credential helper, BEFORE the subcommand, only when asked', () => {
+    // Multi-user non-admin clones must not borrow the server account's git sign-in.
+    // `-c` is a global option: after `clone` git would read it as an unknown flag.
+    expect(
+      buildCloneArgs({ repository: 'https://example.com/r.git', destination: 'd', withoutCredentialHelpers: true })
+    ).toEqual(['-c', 'credential.helper=', 'clone', '--', 'https://example.com/r.git', 'd']);
+    expect(buildLsRemoteArgs('https://example.com/r.git', { withoutCredentialHelpers: true })).toEqual([
+      '-c',
+      'credential.helper=',
+      'ls-remote',
+      '--symref',
+      '--',
+      'https://example.com/r.git',
+    ]);
+    // Absent or false leaves the argv exactly as it was before the option existed.
+    expect(buildCloneArgs({ repository: 'r', destination: 'd', withoutCredentialHelpers: false })[0]).toBe('clone');
+    expect(buildLsRemoteArgs('r', {})[0]).toBe('ls-remote');
+  });
 });
 
 describe('gitNonInteractiveEnv', () => {