v0.15.7 — fix(task): `task need --json` emitted ZERO BYTES for the common case (DIVE-1930)

5dive task need --json returned nothing at all — not a smaller object, not an error, zero bytes — for any gate filed without a matching precedent, which is the overwhelming majority of them. Measured on the rolled 0.15.6 binary.

• one field killed the whole envelope. precedent_ref:(($pr|select(length>0))|tonumber? // null): with no precedent $pr is empty, select yields empty, and because the // null bound to tonumber? instead of to the whole expression, empty | (tonumber? // null) stayed empty and propagated OUT of the object constructor. jq does not build a smaller object in that situation; it builds nothing.
• the discriminator is where // null BINDS, not whether a pipe follows select. (($x|select(length>0)) // null) is safe and (($x|select(length>0)|tonumber?) // null) is safe — both enclose the empty. Only (($x|select(length>0))|tonumber? // null) leaves it uncaught. map(select(...)) and [ ... | select(...) ] are comprehensions and never at risk. The fix is one closing paren.
• it was a ONE-line point fix, not a 19-site sweep. The ticket was scoped from grep hits on select(length>0); enumerating the 23 sites in src/ by GUARD SHAPE instead found six safe // null forms, comprehensions, two streaming into jq -cs where empty correctly yields [], one map(select(...))|length false positive, and exactly one defect. Editing the other eighteen would have been eighteen chances to introduce the bug being removed.
• the only case that worked was the rare one, which is why it survived. A gate WITH a precedent rendered fine. Its regression test asserts both directions, so "fix" by deleting the field fails too — and against the pre-fix line 5 of 6 assertions fail, the sole pass being that rare path.
• a shape guard grep rejects the dangerous binding coming back.
• the goal path could never have caught it: it captured the envelope into a gate_json it never read. A capture that is never inspected reads at review time like a checked result and is not one. Now discarded explicitly, matching the objective path, so the exit status is visibly the only thing that call is trusted for.