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/config/schema/agent-definitions.ts

@@ -0,0 +1,5 @@
+import { z } from "zod"
+
+export const AgentDefinitionPathSchema = z.string()
+
+export const AgentDefinitionsConfigSchema = z.array(AgentDefinitionPathSchema).optional()

src/config/schema/oh-my-opencode-config.ts

@@ -1,6 +1,7 @@
 import { z } from "zod"
 import { AnyMcpNameSchema } from "../../mcp/types"
 import { BuiltinSkillNameSchema } from "./agent-names"
+import { AgentDefinitionsConfigSchema } from "./agent-definitions"
 import { AgentOverridesSchema } from "./agent-overrides"
 import { BabysittingConfigSchema } from "./babysitting"
 import { BackgroundTaskConfigSchema } from "./background-task"
@@ -29,6 +30,8 @@ export const OhMyOpenCodeConfigSchema = z.object({
   new_task_system_enabled: z.boolean().optional(),
   /** Default agent name for `oh-my-opencode run` (env: OPENCODE_DEFAULT_AGENT) */
   default_run_agent: z.string().optional(),
+  /** Paths to external agent definition files (.md or .json) */
+  agent_definitions: AgentDefinitionsConfigSchema,
   disabled_mcps: z.array(AnyMcpNameSchema).optional(),
   disabled_agents: z.array(z.string()).optional(),
   disabled_skills: z.array(BuiltinSkillNameSchema).optional(),