feat(agents): implement dynamic Sisyphus prompt system with agent metadata

Introduce a new dynamic prompt generation system for Sisyphus orchestrator
that leverages agent metadata for intelligent delegation. This revives the
dynamic-sisyphus-agent-prompt branch with comprehensive refactoring.

Changes:
- Add AgentPromptMetadata, AgentCategory, AgentCost, DelegationTrigger types
- Create sisyphus-prompt-builder with dynamic prompt generation logic
- Add AGENT_PROMPT_METADATA exports to all agent modules (oracle, librarian,
  explore, frontend-ui-ux-engineer, document-writer, multimodal-looker)
- Refactor sisyphus.ts to use buildDynamicSisyphusPrompt()
- Add AvailableAgent type export for type safety

This enables Sisyphus to make intelligent agent selection decisions based on
agent capabilities, costs, and delegation triggers, improving orchestration
efficiency.

🤖 Generated with assistance of OhMyOpenCode
(https://github.com/code-yeongyu/oh-my-opencode)

src/agents/explore.ts

@@ -1,7 +1,28 @@
 import type { AgentConfig } from "@opencode-ai/sdk"
+import type { AgentPromptMetadata } from "./types"
 
 const DEFAULT_MODEL = "opencode/grok-code"
 
+export const EXPLORE_PROMPT_METADATA: AgentPromptMetadata = {
+  category: "exploration",
+  cost: "FREE",
+  promptAlias: "Explore",
+  keyTrigger: "2+ modules involved → fire `explore` background",
+  triggers: [
+    { domain: "Explore", trigger: "Find existing codebase structure, patterns and styles" },
+  ],
+  useWhen: [
+    "Multiple search angles needed",
+    "Unfamiliar module structure",
+    "Cross-layer pattern discovery",
+  ],
+  avoidWhen: [
+    "You know exactly what to search",
+    "Single keyword/pattern suffices",
+    "Known file location",
+  ],
+}
+
 export function createExploreAgent(model: string = DEFAULT_MODEL): AgentConfig {
   return {
     description: