~anthropic/claude-sonnet-latest → B (6:4)

Side B fixes a real off-by-one/indexing bug (0-based vs 1-based thread_post_index mismatch used in URLs), removes a dead conditional in the template, and adds regression tests plus doc clarifications across three files, giving concrete lasting correctness value. Side A merely removes a redundant early-return guard that was already dead code, a smaller and lower-impact cleanup with a single adjusted test.