M28-C: Tool and skill filtering for sub-agents

[bug-ops]

Parent: #709

Summary

Implement tool access control and skill filtering for sub-agents based on their definition's ToolPolicy and SkillFilter.

Scope

• FilteredToolExecutor: wraps ErasedToolExecutor, enforces AllowList/DenyList
• SkillFilter application: glob matching on skill names for include/exclude
• Filtered skills injected into sub-agent system prompt at spawn time
• ToolPolicy enforcement at executor level (not prompt level)

Acceptance Criteria

• AllowList: sub-agent can only call listed tools
• DenyList: sub-agent can call all tools except listed
• InheritAll: sub-agent inherits parent tool access
• Denied tool call returns clear error message to sub-agent LLM
• SkillFilter with glob patterns (e.g., "rust-*") filters correctly
• Empty include list = all skills allowed
• Unit tests for FilteredToolExecutor and SkillFilter