feat(agents): add agent definitions file loader and opencode.json reader

- Add loadAgentDefinitions() for explicit file path loading (.md/.json/.jsonc)
- Add readOpencodeConfigAgents() for independent opencode.json(c) reading
- Extract parseMarkdownAgentFile() from loader.ts for reuse
- Refactor loader.ts to use extracted parser (-50 LOC)
- Add comprehensive test coverage (13 tests for definitions loader, 10 tests for opencode reader)
- Support inline agents + agent_definitions paths in opencode.json(c)
- Inline agents override definition-file agents (correct precedence)

Part of agent definitions enhancement (Wave 2/3)

src/features/claude-code-agent-loader/agent-definitions-loader.ts

@@ -0,0 +1,98 @@
+import { existsSync, readFileSync } from "fs"
+import { basename, extname } from "path"
+import { parseFrontmatter } from "../../shared/frontmatter"
+import { log } from "../../shared/logger"
+import { parseJsonAgentFile } from "./json-agent-loader"
+import { mapClaudeModelToOpenCode } from "./claude-model-mapper"
+import type { AgentScope, AgentFrontmatter, ClaudeCodeAgentConfig, LoadedAgent } from "./types"
+
+function parseToolsConfig(toolsStr?: string): Record<string, boolean> | undefined {
+  if (!toolsStr) return undefined
+
+  const tools = toolsStr.split(",").map((t) => t.trim()).filter(Boolean)
+  if (tools.length === 0) return undefined
+
+  const result: Record<string, boolean> = {}
+  for (const tool of tools) {
+    result[tool.toLowerCase()] = true
+  }
+  return result
+}
+
+export function parseMarkdownAgentFile(filePath: string, scope: AgentScope): LoadedAgent | null {
+  try {
+    if (!existsSync(filePath)) {
+      return null
+    }
+
+    const content = readFileSync(filePath, "utf-8")
+    const { data, body } = parseFrontmatter<AgentFrontmatter>(content)
+
+    const agentName = basename(filePath, ".md")
+    const name = data.name || agentName
+    const originalDescription = data.description || ""
+
+    const formattedDescription = `(${scope}) ${originalDescription}`
+
+    const mappedModelOverride = mapClaudeModelToOpenCode(data.model)
+    const modelString = mappedModelOverride
+      ? `${mappedModelOverride.providerID}/${mappedModelOverride.modelID}`
+      : undefined
+
+    const config: ClaudeCodeAgentConfig = {
+      description: formattedDescription,
+      mode: data.mode || "subagent",
+      prompt: body.trim(),
+      ...(modelString ? { model: modelString } : {}),
+    }
+
+    const toolsConfig = parseToolsConfig(data.tools)
+    if (toolsConfig) {
+      config.tools = toolsConfig
+    }
+
+    return {
+      name,
+      path: filePath,
+      config,
+      scope,
+    }
+  } catch {
+    return null
+  }
+}
+
+export function loadAgentDefinitions(
+  paths: string[],
+  scope: AgentScope
+): Record<string, ClaudeCodeAgentConfig> {
+  const result: Record<string, ClaudeCodeAgentConfig> = {}
+
+  for (const filePath of paths) {
+    if (!existsSync(filePath)) {
+      log(`[agent-definitions-loader] File not found, skipping: ${filePath}`)
+      continue
+    }
+
+    const ext = extname(filePath).toLowerCase()
+    let agent: LoadedAgent | null = null
+
+    if (ext === ".md") {
+      agent = parseMarkdownAgentFile(filePath, scope)
+    } else if (ext === ".json" || ext === ".jsonc") {
+      agent = parseJsonAgentFile(filePath, scope)
+    } else {
+      log(`[agent-definitions-loader] Unsupported file extension: ${ext} for ${filePath}`)
+      continue
+    }
+
+    if (!agent) {
+      log(`[agent-definitions-loader] Failed to parse agent file: ${filePath}`)
+      continue
+    }
+
+    result[agent.name] = agent.config
+  }
+
+  return result
+}