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/features/claude-code-agent-loader/json-agent-loader.ts

@@ -1,23 +1,9 @@
 import { existsSync, readFileSync } from "fs"
 import { parseJsoncSafe } from "../../shared/jsonc-parser"
+import { parseToolsConfig } from "../../shared/parse-tools-config"
 import { mapClaudeModelToOpenCode } from "./claude-model-mapper"
 import type { AgentScope, AgentJsonDefinition, ClaudeCodeAgentConfig, LoadedAgent } from "./types"
 
-function parseToolsConfig(tools?: string | string[]): Record<string, boolean> | undefined {
-  if (!tools) return undefined
-
-  const toolsArray = Array.isArray(tools) ? tools : tools.split(",").map((t) => t.trim())
-  const filtered = toolsArray.filter((t) => typeof t === "string" && t.length > 0)
-
-  if (filtered.length === 0) return undefined
-
-  const result: Record<string, boolean> = {}
-  for (const tool of filtered) {
-    result[tool.toLowerCase()] = true
-  }
-  return result
-}
-
 export function parseJsonAgentFile(filePath: string, scope: AgentScope): LoadedAgent | null {
   try {
     if (!existsSync(filePath)) {