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/plugin-config.ts

@@ -9,6 +9,7 @@ import {
   parseJsonc,
   detectPluginConfigFile,
   migrateConfigFile,
+  resolveAgentDefinitionPaths,
 } from "./shared";
 import { migrateLegacyConfigFile } from "./shared/migrate-legacy-config-file";
 import { CONFIG_BASENAME, LEGACY_CONFIG_BASENAME } from "./shared/plugin-identity";
@@ -41,6 +42,7 @@ const PARTIAL_STRING_ARRAY_KEYS = new Set([
   "disabled_commands",
   "disabled_tools",
   "mcp_env_allowlist",
+  "agent_definitions",
 ]);
 
 export function parseConfigPartially(
@@ -139,6 +141,12 @@ export function mergeConfigs(
     ...override,
     agents: deepMerge(base.agents, override.agents),
     categories: deepMerge(base.categories, override.categories),
+    agent_definitions: [
+      ...new Set([
+        ...(base.agent_definitions ?? []),
+        ...(override.agent_definitions ?? []),
+      ]),
+    ],
     disabled_agents: [
       ...new Set([
         ...(base.disabled_agents ?? []),
@@ -250,6 +258,15 @@ export function loadPluginConfig(
   // Load user config first (base). Parse empty config through Zod to apply field defaults.
   const userConfig = loadConfigFromPath(userConfigPath, ctx)
   const userGitMasterOverrides = loadExplicitGitMasterOverrides(userConfigPath)
+
+  if (userConfig?.agent_definitions) {
+    userConfig.agent_definitions = resolveAgentDefinitionPaths(
+      userConfig.agent_definitions,
+      configDir,
+      null
+    )
+  }
+
   let config: OhMyOpenCodeConfig =
     userConfig ?? OhMyOpenCodeConfigSchema.parse({});
 
@@ -257,6 +274,15 @@ export function loadPluginConfig(
   const defaultGitMaster = OhMyOpenCodeConfigSchema.parse({}).git_master
   const projectConfig = loadConfigFromPath(projectConfigPath, ctx);
   const projectGitMasterOverrides = loadExplicitGitMasterOverrides(projectConfigPath)
+
+  if (projectConfig?.agent_definitions) {
+    projectConfig.agent_definitions = resolveAgentDefinitionPaths(
+      projectConfig.agent_definitions,
+      directory,
+      directory
+    )
+  }
+
   if (projectConfig) {
     config = mergeConfigs(config, projectConfig);
   }