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>

src/config/schema.ts

@@ -108,9 +108,8 @@ export const ClaudeCodeConfigSchema = z.object({
 
 export const SisyphusAgentConfigSchema = z.object({
   disabled: z.boolean().optional(),
-  builder_enabled: z.boolean().optional(),
+  default_builder_enabled: z.boolean().optional(),
   planner_enabled: z.boolean().optional(),
-  replace_build: z.boolean().optional(),
   replace_plan: z.boolean().optional(),
 })