Add versioning contract: semver policy, breaking change sections, schema_version guard

- CHANGELOG: add ### Breaking Changes section to all versions (None for 1.1–1.3)
- README: add Versioning section with semver table and floating tag explanation
- schema/config-schema.json: add schema_version field (const: 1, optional)
- lib/config.sh: fail fast if schema_version is present and unsupported

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

schema/config-schema.json

@@ -5,6 +5,11 @@
   "type": "object",
   "required": ["josh", "targets", "bot"],
   "properties": {
+    "schema_version": {
+      "type": "integer",
+      "const": 1,
+      "description": "Schema version. Bump when config fields are removed or renamed (matches josh-sync major version)."
+    },
     "josh": {
       "type": "object",
       "required": ["proxy_url", "monorepo_path"],