fix(framework/tools): wrapper body-safety + login-resolution hardening (#559, #560)

#559 — Markdown body safety / eval removal:
- Add test-issue-create-body-safety.sh: feeds a hostile Markdown body
  ($(...), backticks, quotes, $vars, pipes) through issue-create.sh and
  asserts no command substitution runs and the body reaches tea verbatim.
- Convert issue-comment.sh from unquoted $(get_gitea_repo_args) word-splitting
  to an argv array with an explicit loud login-resolution error.
- Confirmed: zero eval usages remain across tools/git/*.sh; the other
  body-carrying wrappers (issue-create, pr-create, issue-edit, issue-assign)
  already use argv arrays.

#560 — host-derived Gitea login + loud failure:
- detect-platform.sh: add print_gitea_login_diagnostic and emit it on the
  get_gitea_login_for_host failure path (stderr only) — names the unresolved
  host, lists available tea logins, and gives the GITEA_LOGIN override +
  tea-login-add fix. Replaces the previous silent failure.
- Extend test-gitea-login-resolution.sh: assert the diagnostic fires and lists
  logins, login is derived from origin host for both mosaicstack and usc (scoped
  second tea mock), and a valid GITEA_LOGIN override is honored.

Also gitignore the .mosaic-test-work/ shell-harness scratch dir.
Scope: wrapper surface only. All wrapper test harnesses pass locally.

.gitignore

@@ -12,3 +12,6 @@ docs/reports/
 
 # Step-CA dev password — real file is gitignored; commit only the .example
 infra/step-ca/dev-password
+
+# Scratch dirs created by the framework git-wrapper shell test harnesses
+.mosaic-test-work/