fix(sdd): add Spectral ruleset so contract-validate passes

Spectral v6 ships no implicit ruleset — the CI job exited 'no ruleset found'.
Adds .spectral.yaml (extends spectral:oas, documentation-only warnings relaxed
for design-time stubs), adds operation tags to the _example contract so it lints
clean (0 results), and aligns the api-contract-stub note.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.spectral.yaml

@@ -0,0 +1,15 @@
+# Spectral ruleset for OpenAPI contract linting (SDD api-contract files).
+# Spectral v6 ships no implicit ruleset — this enables the built-in OpenAPI rules.
+# Used by .gitea/workflows/sdd-gate.yml (contract-validate) and locally:
+#   npx @stoplight/spectral-cli lint <contract>.yaml
+extends: ["spectral:oas"]
+
+rules:
+  # Design-time SDD stubs are not full published API docs — relax the documentation-completeness
+  # warnings that would otherwise fire on a focused contract. The structural/correctness rules
+  # (oas3-schema, valid $refs, duplicate operationId, etc.) stay on.
+  info-contact: off
+  info-description: off
+  operation-description: off
+  operation-tag-defined: off
+  oas3-unused-component: off