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/config/schema/agent-definitions.ts

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