Follow-up to #1095. The reactions auto-enrichment shipped, but on busy chats the strictly-serial per-resource fetches in EnrichReactions, ExpandThreadReplies, and merge_forward expansion stretched the command's wall time above 14s — enough that wrapper agents (30–60s wall-clock budgets) saw timeouts even though the CLI itself never errored. This PR parallelizes all three with the same bounded-concurrency pattern, batches the follow-up contact-API sender resolution so it doesn't fan back out into a serial stall, and fixes two correctness bugs that surfaced during review. Scoped to convert_lib/{reactions,thread,merge,content_convert}.go + tests + the 4 shortcut Execute hooks + the reference doc.
Change-Id: I0206d10ad204382170bd42aec67f82578923736e
