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/agent-resolver.test.ts

@@ -12,8 +12,8 @@
  * R6: No duplicate agent names in output
  * R7: Malformed agent entries (null, missing name, non-string name, whitespace-only) are skipped gracefully
  */
-const { describe, test, expect, mock } = require("bun:test")
-const { resolveCallableAgents } = require("./agent-resolver")
+const { describe, test, expect, mock, beforeEach } = require("bun:test")
+const { resolveCallableAgents, clearCallableAgentsCache } = require("./agent-resolver")
 const { ALLOWED_AGENTS } = require("./constants")
 
 function createMockClient(agents: Array<Record<string, unknown>>) {
@@ -33,6 +33,10 @@ function createFailingClient(error: Error = new Error("API unavailable")) {
 }
 
 describe("resolveCallableAgents", () => {
+  beforeEach(() => {
+    clearCallableAgentsCache()
+  })
+
   describe("#given the SDK returns agents successfully", () => {
     describe("#when only built-in agents exist", () => {
       test("#then every ALLOWED_AGENT appears in the result", async () => {