tools/breenix-runs/DESIGN.md cites the failure-capture plan by its pre-commit filename in 5 places

[ryanbreen]

The failure-trace-capture plan is now committed at
docs/planning/green-program/failure-capture/PLAN-2026-09-05.md (by PR-3 of
that program, branch capture/ftc-pr3-bxcap-emitter). It previously lived
outside the tree.

tools/breenix-runs/DESIGN.md cites it by its pre-commit filename,
FAILURE-TRACE-CAPTURE-PLAN-2026-09-05.md, in 5 places (verified by
git grep -c against origin/main at 5121cfca). Those citations now name a
file that exists, but by a name that does not, so a reader following one has
to guess.

Left out of the PR-3 branch deliberately: tools/breenix-runs/DESIGN.md is
being actively rewritten in the tools/breenix-runs lane -- it changed
between ee48a4a1 and 5121cfca -- and a 5-line citation edit from another
branch would produce a merge conflict for no functional benefit. The right
place to make it is that lane's next round, or a standalone one-file commit on
main.

Fix: replace the 5 occurrences with
docs/planning/green-program/failure-capture/PLAN-2026-09-05.md.