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>

.gitea/workflows/spec-check.yml

@@ -0,0 +1,20 @@
+name: spec check
+
+# Builds the Lean semantic master spec under spec/.
+# This is an INTERNAL well-formedness gate (does the contract type-check?),
+# NOT a spec-to-implementation conformance gate — implementations align to the
+# contract by human review, not by CI. See repo README.
+
+on:
+  push:
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  spec-check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+      - uses: leanprover/lean-action@v1
+        with:
+          lake-package-directory: spec