fix: use portable syntax to strip trailing comma in build_fields

[mc0de]

This PR fixes two bugs that together made --field completely non-functional.

Bug 1: add_field always exited early

add_field guards against being called outside of an embed context:

[[ -z "${embedding}" ]] && exit;

However, in the argument parser, embedding=1 was set after calling add_field:

--field*) add_field "${2}"; embedding=1; shift; shift;;

This meant that when --field was the first (or only) embed-related flag, embedding was still unset when add_field ran, causing it to exit immediately without appending anything to fields.

Fix: swapped the order so embedding=1 is set before add_field is called.

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Bug 2: ${fields::-1} silently fails on bash < 4.2

build_fields used ${fields::-1} to strip the trailing comma from the accumulated field entries. This negative-length substring expansion requires bash 4.2+ and silently produces no output on macOS, which ships with bash 3.2 by default.

As a result, _fields was always assigned an empty string inside build_embed, and the "fields" key never appeared in the embed JSON — making --field a no-op on any system running bash < 4.2.

Fix: replaced ${fields::-1} with ${fields%,}, which strips the trailing comma using suffix pattern removal — a POSIX-compatible feature available in all bash versions.