decisions.md is 464 kB with 19 kB rows; split into one file per decision with an index #164

Severity: Low-medium (process debt). Review finding D9; deferred by #157. Re-verified at 1c4e71e.

docs/decisions/decisions.md is 464,094 bytes. Its longest row is 19,488 characters (D-142, now line 164), and it carries the SECOND FINDING plus several inline CORRECTION passages in one table cell. Any grep that hits it dumps ~19 kB, and a reader cannot tell a decision's current status without reading the whole cell.

Reproduce: wc -c docs/decisions/decisions.md; awk '{if(length>m){m=length;l=NR}} END{print m, l}' docs/decisions/decisions.md

Fix (own lane): one file per decision with machine-checkable Status: / Superseded-by: headers, plus a generated index, gated by the existing truth-lag pattern (hack/docs/truthlag_pins_test.sh).