fix(agents): address all PR #2299 code review findings

Blocking fixes:
- B1: Return empty restrictions for unknown/custom agents instead of
  EXPLORATION_AGENT_DENYLIST, allowing custom agents full tool access
- B2: Use Object.create(null) consistently across all 5 agent-loading
  result objects to prevent prototype pollution
- B3: Add code comment documenting custom agent bash access trust model
- B4: Mock getOpenCodeConfigDir in opencode-config-agents-reader tests
  to prevent global config dir leakage

Non-blocking fixes:
- N1: Use resolveAgentDefinitionPaths with project boundary enforcement
  in opencode-config-agents-reader for path containment
- N2: Add session-scoped 30s TTL cache to resolveCallableAgents to
  avoid redundant SDK IPC calls per tool invocation
- N3: Extract shared parseToolsConfig into src/shared/parse-tools-config.ts
  replacing 4 duplicated local implementations
- N4: Add .min(1) to AgentDefinitionPathSchema rejecting empty paths
- N5: Add resolve-agent-definition-paths.test.ts covering tilde expansion,
  relative paths, boundary enforcement, and null containmentDir
- N6: Validate agent mode against allowed values instead of bare type
  assertion in opencode-config-agents-reader

src/tools/call-omo-agent/tools.test.ts

@@ -1,5 +1,6 @@
 const { beforeEach, describe, test, expect, mock } = require("bun:test")
 const { createCallOmoAgent } = require("./tools")
+const { clearCallableAgentsCache } = require("./agent-resolver")
 
 type PluginInput = { client: any; directory: string }
 type BackgroundManager = {
@@ -72,6 +73,7 @@ const toolCtx = {
 }
 
 beforeEach(() => {
+  clearCallableAgentsCache()
   assertCanSpawnMock.mockClear()
   reserveSubagentSpawnMock.mockClear()
   reserveCommitMock.mockClear()