feat(agents): add agent_definitions schema, eager path resolution, and JSON agent loader

Wave 1 of agent definitions enhancement (PR #2299):

Schema & Configuration:
- Add agent_definitions field to oh-my-opencode config schema
- Support list of file paths to .md or .json agent definition files
- Add to PARTIAL_STRING_ARRAY_KEYS for Set-union merge semantics
- Implement eager path resolution in loadPluginConfig() before merging

Path Resolution:
- Create resolve-agent-definition-paths.ts helper
- User-level paths resolve from ~/.config/opencode/ (no containment)
- Project-level paths resolve from project root (with containment check)
- Homedir expansion, absolute/relative path handling

JSON Agent Loader:
- Create parseJsonAgentFile() for .json/.jsonc agent definitions
- Validate required fields (name, prompt)
- Support tools as string (comma-separated) or array
- Map model via mapClaudeModelToOpenCode()
- Comprehensive test suite (7 test cases, all passing)

Type Extensions:
- Extend AgentScope: add 'definition-file' and 'opencode-config'
- Add AgentJsonDefinition interface for JSON agent schema

All automated checks passing:
- lsp_diagnostics clean on all changed files
- json-agent-loader.test.ts: 7/7 passing
- Full typecheck: zero new errors
- QA evidence saved to .sisyphus/evidence/

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

@@ -0,0 +1,67 @@
+import { existsSync, readFileSync } from "fs"
+import { parseJsoncSafe } from "../../shared/jsonc-parser"
+import { mapClaudeModelToOpenCode } from "./claude-model-mapper"
+import type { AgentScope, AgentJsonDefinition, ClaudeCodeAgentConfig, LoadedAgent } from "./types"
+
+function parseToolsConfig(tools?: string | string[]): Record<string, boolean> | undefined {
+  if (!tools) return undefined
+
+  const toolsArray = Array.isArray(tools) ? tools : tools.split(",").map((t) => t.trim())
+  const filtered = toolsArray.filter((t) => typeof t === "string" && t.length > 0)
+
+  if (filtered.length === 0) return undefined
+
+  const result: Record<string, boolean> = {}
+  for (const tool of filtered) {
+    result[tool.toLowerCase()] = true
+  }
+  return result
+}
+
+export function parseJsonAgentFile(filePath: string, scope: AgentScope): LoadedAgent | null {
+  try {
+    if (!existsSync(filePath)) {
+      return null
+    }
+
+    const content = readFileSync(filePath, "utf-8")
+    const { data } = parseJsoncSafe<AgentJsonDefinition>(content)
+
+    if (!data) {
+      return null
+    }
+
+    if (!data.name || !data.prompt) {
+      return null
+    }
+
+    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: data.prompt.trim(),
+      ...(modelString ? { model: modelString } : {}),
+    }
+
+    const toolsConfig = parseToolsConfig(data.tools)
+    if (toolsConfig) {
+      config.tools = toolsConfig
+    }
+
+    return {
+      name: data.name,
+      path: filePath,
+      config,
+      scope,
+    }
+  } catch {
+    return null
+  }
+}