fix(framework): detect-platform get_gitea_token fail-loud on absent per-slot token (Patch 2b) (#890)

Co-authored-by: jason.woltje <jason@diversecanvas.com>
Co-committed-by: jason.woltje <jason@diversecanvas.com>

packages/mosaic/framework/tools/git/detect-platform.sh

@@ -511,7 +511,11 @@ get_gitea_token() {
     #    (pr-create, issue-create, …) authors under the right identity — matching the
     #    git credential helper. Backward-compatible: nothing resolvable → shared logic below.
     local _ident="${MOSAIC_GIT_IDENTITY:-}"
-    [[ -z "$_ident" ]] && _ident="$(git config --get mosaic.gitIdentity 2>/dev/null || true)"
+    local _ident_src="MOSAIC_GIT_IDENTITY"
+    if [[ -z "$_ident" ]]; then
+        _ident="$(git config --get mosaic.gitIdentity 2>/dev/null || true)"
+        _ident_src="git config mosaic.gitIdentity"
+    fi
     if [[ -n "$_ident" ]]; then
         local _idpfx=""
         case "$host" in
@@ -524,6 +528,15 @@ get_gitea_token() {
                 cat "$_idtok"
                 return 0
             fi
+            # FAIL LOUD: an explicit git identity was requested for a recognized Gitea host,
+            # but no per-slot token exists for THAT identity. Refuse to fall through to the
+            # shared/default credential loader below — silently borrowing another slot's token
+            # would post PRs/issues/reviews under the WRONG agent (e.g. rev2's review attributed
+            # to coder3), corrupting Gate-16 author≠reviewer separation. Hard-stop instead so the
+            # caller aborts loudly rather than acting as the wrong identity.
+            echo "Error: git identity '$_ident' requested (via $_ident_src) for host '$host', but no per-slot token at $_idtok." >&2
+            echo "       Refusing to borrow another slot's token. Provision the per-slot token, or unset the identity to use shared credentials." >&2
+            return 1
         fi
     fi