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.

packages/mosaic/framework/tools/git/issue-comment.sh

@@ -53,7 +53,15 @@ if [[ "$PLATFORM" == "github" ]]; then
     gh issue comment "$ISSUE_NUMBER" --body "$COMMENT"
     echo "Added comment to GitHub issue #$ISSUE_NUMBER"
 elif [[ "$PLATFORM" == "gitea" ]]; then
-    tea issue comment "$ISSUE_NUMBER" "$COMMENT" $(get_gitea_repo_args)
+    # Build the invocation as an argv array (not unquoted $(get_gitea_repo_args)
+    # word-splitting) so the comment body — including Markdown backticks, $(...),
+    # and quotes — is passed verbatim and never re-split or shell-evaluated.
+    REPO_SLUG=$(get_repo_slug)
+    GITEA_LOGIN_NAME=$(get_gitea_login) || {
+        echo "Error: could not resolve a Gitea login for this repo; cannot comment on issue #$ISSUE_NUMBER." >&2
+        exit 1
+    }
+    tea issue comment "$ISSUE_NUMBER" "$COMMENT" --repo "$REPO_SLUG" --login "$GITEA_LOGIN_NAME"
     echo "Added comment to Gitea issue #$ISSUE_NUMBER"
 else
     echo "Error: Unknown platform"