Revert "docs(devlog): add missing devlog entries for Steps 5, 6.3, and Step 6 completion"

This reverts commit 7249b33fca.

Removes the four devlog entry records added by that commit
(step-5-completion, step-6-surface-discovery,
contract-5-3-accepted-baseline, step-6-completion) and restores
.labyricorn/devlog/contents.lr to its prior title and summary.

The annotated tags contract-5.3-accepted-baseline and step-6-complete,
and all underlying implementation, documentation and evidence, are
unchanged. Only .labyricorn/devlog/ publishing records are affected;
the uncommitted project identity record and other working-tree changes
are deliberately left out of this commit.

python devlog_editor.py --validate passes; git diff --check clean.

.labyricorn/devlog/contents.lr

@@ -2,6 +2,6 @@ _model: devlog
 ---
 schema_version: 1
 ---
-title: XZBT-NGN Exhibit Engine development log
+title: Labyricorn Project Template development log
 ---
-summary: Milestones and development log entries for the XZBT-NGN Exhibit Engine project.
+summary: Milestones and development log entries for the Labyricorn project template.