artifact-completeness-benchmark: evidence tie-in — the completeness pillar for the paper

[tcballard]

Sub-issue of #299 (artifact-completeness-benchmark, ADR-093). Depends on #301 (results).

Corpus: Initiative 3 of the roadmap; composes with rac/roadmaps/future/external-benchmark-evidence.md and the decision-grounding-paper (#293).

Scope:

• Fold the SWE-CompletenessBench residual result into the decision-grounding-paper as the completeness pillar, beside SWE-DecisionBench's adherence pillar: DecisionBench measures the lift recorded decisions give an agent; CompletenessBench measures the completeness of the record itself.
• Position in the SWE-* lineage as the fourth node (resolution → context → decision-adherence → completeness), citing the family without implying endorsement.
• Report the residual honestly whichever way it falls (a large tacit gap is a valid finding, not one to suppress).
• Same publication gate as the paper: external publication waits on GATE-1 (decision-grounding-paper: clearance and submission — GATE-1, arXiv, peer venue #296).

Tick on #299 when the result is written up as the paper's completeness pillar.