feat(semantic-spacetime): add format-compliant M1 skill core

Add the semantic-spacetime skill skeleton: a thin SKILL.md router with
triggers/anti-triggers, Load By Need, Quick Start, Related Skills, gotchas,
and exit conditions; a human-facing README; MIT license; deep
provenance-marked theory references (foundations, glossary, bibliography);
the versioned sst-model-v1 template pair; a 6-case schema-v1 eval manifest;
the root README catalog entry; regenerated marketplace and llms artifacts;
and the skill-triggers row.

Co-authored-by: factory-droid[bot] <138933559+factory-droid[bot]@users.noreply.github.com>

.codex-plugin/plugin.json

@@ -131,6 +131,7 @@
     "./restic",
     "./secure-software-engineering",
     "./security-audit-methodology",
+    "./semantic-spacetime",
     "./seo-audit",
     "./site-reliability-engineering",
     "./slack",