chore(ci): docs build CI workflow (REQ-032, D-046, D-051, G-016)

Add .gitea/workflows/docs-build.yml: on push, go-test job runs go test ./...
(lexicon firewall + all x/* tests), then docs-build job (needs: go-test per
G-016 firewall-gates-docs-build) installs mkdocs + mkdocs-material, runs
mkdocs build, uploads site/ as a CI artifact. go.mod unchanged (Python deps
isolated to the docs-build job; G-006 intact). Full Gitea Pages publishing
deferred per D-051 (no hosting target configured).

.gitignore: add site/ (mkdocs build output; never committed).

Verification: YAML parses; docs-build needs go-test; go test ./... green;
go.mod unchanged; local mkdocs build succeeds (site/ produced).

---ci---
project: oy
phase: 3
milestone: v0.4
status: execute
tag_base: v0.3.x
milestone_type: nfr
reqs: [REQ-032]
---/ci---

.gitea/workflows/docs-build.yml

@@ -0,0 +1,55 @@
+# OpenYield docs build CI (REQ-032, D-046 forward-reference, D-051, G-016).
+#
+# Runs the lexicon firewall (go test ./...) AND builds the MkDocs Material docs
+# site on every push. The docs-build job DEPENDS on go-test (G-016 binding:
+# firewall-gates-docs-build — a lexicon violation blocks the docs build so no
+# false-green docs artifact is produced from a repo with a firewall failure).
+#
+# Scope (chore, not feat: per D-001 refinement-only filter):
+#   - go-test job: setup Go 1.22, run `go test ./...` (lexicon firewall + all
+#     x/* tests + the v0.4 cross-const test). Zero external Go deps (G-006).
+#   - docs-build job: setup Python, pip install mkdocs + mkdocs-material
+#     (build-only Python deps, ISOLATED to this job — go.mod is NOT modified),
+#     run `mkdocs build` (produces site/), upload site/ as a CI artifact.
+#
+# Out of scope (deferred per D-051): full Gitea Pages publishing. v0.4 ships
+# build + artifact only; a hosting target is not configured.
+#
+# Triggers: on push (all branches) so the firewall + docs build are checked
+# on every change, not just on main.
+
+name: docs-build
+on:
+  push:
+
+jobs:
+  go-test:
+    name: go test ./... (lexicon firewall + all x/* tests)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-go@v5
+        with:
+          go-version: '1.22'
+      - name: go test ./...
+        run: go test ./...
+
+  docs-build:
+    name: mkdocs build (docs site artifact)
+    runs-on: ubuntu-latest
+    needs: go-test  # G-016: firewall-gates-docs-build (no false-green docs build)
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+      - name: install mkdocs + mkdocs-material
+        run: pip install mkdocs mkdocs-material
+      - name: mkdocs build
+        run: mkdocs build
+      - name: upload site/ artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: docs-site
+          path: site/
+          retention-days: 14

.gitignore

@@ -2,3 +2,5 @@
 .env.secrets
 .env.*
 .ciagent/.env.secrets
+# MkDocs build output (REQ-032 CI produces site/ as an artifact; never commit it)
+site/