Enables passing skills=["skill-name"] in the task dict to inject skill content into subagent system prompts.

What this does

• Subagents can now load skills — closes a real gap where parent knowledge doesn't transfer
• Skill content is injected into the child system prompt under a "Loaded skills" section
• Gracefully skips missing skills (no crash)

Implementation

• _load_skill_content(name) searches skills/ and ~/.hermes/skills/ for SKILL.md
• _build_child_system_prompt() accepts optional skills list
• delegate_task() passes t.get("skills") through to child agent construction

Tests

5 tests covering:

• Skill content lookup (found/not found)
• Prompt injection
• Missing skills silently skipped
• No crash when skills omitted

All existing delegate tests still pass.

This PR addresses one of the three items requested in #3387 review. Two more PRs coming:

1. ✅ Skill injection (this PR)
2. MCP workspace guard
3. Configurable max_depth