refactor: rename builder_enabled to default_builder_enabled and remove replace_build (#251)

- Renamed sisyphus_agent.builder_enabled to default_builder_enabled for clarity
- Removed sisyphus_agent.replace_build option entirely
- Default build agent is now always demoted to subagent mode when Sisyphus is enabled
- Updated schema and regenerated JSON schema
- Updated all documentation (EN, KO, JA, ZH-CN)

BREAKING CHANGE: Configuration migration required for users using builder_enabled or replace_build options.

Closes #250

Co-authored-by: sisyphus-dev-ai <sisyphus-dev-ai@users.noreply.github.com>

assets/oh-my-opencode.schema.json

@@ -1352,15 +1352,12 @@
         "disabled": {
           "type": "boolean"
         },
-        "builder_enabled": {
+        "default_builder_enabled": {
           "type": "boolean"
         },
         "planner_enabled": {
           "type": "boolean"
         },
-        "replace_build": {
-          "type": "boolean"
-        },
         "replace_plan": {
           "type": "boolean"
         }