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/shared/index.ts

@@ -17,6 +17,7 @@ export * from "./claude-config-dir"
 export * from "./jsonc-parser"
 export * from "./migration"
 export * from "./opencode-config-dir"
+export * from "./resolve-agent-definition-paths"
 export type {
   OpenCodeBinaryType,
   OpenCodeConfigDirOptions,