chore: scaffold monorepo with spec/ Lean project and governance docs

- spec/: freshly initialized Lean library project (lake init, v4.31.0), no deps
- .gitea/workflows/spec-check.yml: CI running lake build on spec/ (well-formedness gate, not a spec-to-impl gate)
- README.md: monorepo overview + the 5 "constitution" rules positioning spec/ as upstream semantic master
- CLAUDE.md: global agent manual
- spec/README.md: contract writing conventions (prose+type, PINNED/OPEN/ADR tags, no sorry)
- docs/adr/0001-0004: system-level decision records (retrieved from main)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,6 @@
+# Lean / Lake build artifacts (spec/ has its own .gitignore too)
+.lake/
+**/.lake/
+
+# OS / editor
+.DS_Store