feat: add Builder-Sisyphus agent with independent toggle options (#214)

* feat: add Builder-Sisyphus agent with independent toggle options

- Add Builder-Sisyphus agent (disabled by default) for build mode
- Implement independent configuration for Builder/Planner-Sisyphus agents
- Add replace_build and replace_plan options to control agent demotion
- Update schema to support new configuration options
- Update README with comprehensive configuration documentation

Addresses #212: Users can now keep default OpenCode build mode alongside Builder-Sisyphus

* docs: add OpenCode permalinks and update multilingual README files

- Add OpenCode source code permalinks to build-prompt.ts (@see tags)
- Update README.ja.md with Builder-Sisyphus documentation
- Update README.ko.md with Builder-Sisyphus documentation
- Update README.zh-cn.md with Builder-Sisyphus documentation

Permalinks reference:
- Build mode switch: build-switch.txt
- Build agent definition: agent.ts#L118-L125
- Default permissions: agent.ts#L57-L68

---------

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

src/agents/build-prompt.ts

@@ -0,0 +1,68 @@
+/**
+ * OpenCode's default build agent system prompt.
+ *
+ * This prompt enables FULL EXECUTION mode for the build agent, allowing file
+ * modifications, command execution, and system changes while focusing on
+ * implementation and execution.
+ *
+ * Inspired by OpenCode's build agent behavior.
+ * 
+ * @see https://github.com/sst/opencode/blob/6f9bea4e1f3d139feefd0f88de260b04f78caaef/packages/opencode/src/session/prompt/build-switch.txt
+ * @see https://github.com/sst/opencode/blob/6f9bea4e1f3d139feefd0f88de260b04f78caaef/packages/opencode/src/agent/agent.ts#L118-L125
+ */
+export const BUILD_SYSTEM_PROMPT = `<system-reminder>
+# Build Mode - System Reminder
+
+BUILD MODE ACTIVE - you are in EXECUTION phase. Your responsibility is to:
+- Implement features and make code changes
+- Execute commands and run tests
+- Fix bugs and refactor code
+- Deploy and build systems
+- Make all necessary file modifications
+
+You have FULL permissions to edit files, run commands, and make system changes.
+This is the implementation phase - execute decisively and thoroughly.
+
+---
+
+## Responsibility
+
+Your current responsibility is to implement, build, and execute. You should:
+- Write and modify code to accomplish the user's goals
+- Run tests and builds to verify your changes
+- Fix errors and issues that arise
+- Use all available tools to complete the task efficiently
+- Delegate to specialized agents when appropriate for better results
+
+**NOTE:** You should ask the user for clarification when requirements are ambiguous,
+but once the path is clear, execute confidently. The goal is to deliver working,
+tested, production-ready solutions.
+
+---
+
+## Important
+
+The user wants you to execute and implement. You SHOULD make edits, run necessary
+tools, and make changes to accomplish the task. Use your full capabilities to
+deliver excellent results.
+</system-reminder>
+`
+
+/**
+ * OpenCode's default build agent permission configuration.
+ *
+ * Allows the build agent full execution permissions:
+ * - edit: "ask" - Can modify files with confirmation
+ * - bash: "ask" - Can execute commands with confirmation  
+ * - webfetch: "allow" - Can fetch web content
+ *
+ * This provides balanced permissions - powerful but with safety checks.
+ * 
+ * @see https://github.com/sst/opencode/blob/6f9bea4e1f3d139feefd0f88de260b04f78caaef/packages/opencode/src/agent/agent.ts#L57-L68
+ * @see https://github.com/sst/opencode/blob/6f9bea4e1f3d139feefd0f88de260b04f78caaef/packages/opencode/src/agent/agent.ts#L118-L125
+ */
+export const BUILD_PERMISSION = {
+  edit: "ask" as const,
+  bash: "ask" as const,
+  webfetch: "allow" as const,
+}