Add Agent Skills runtime support

[vilsonrodrigues]

Summary

• Add Agent Skills discovery/runtime support with explicit paths, glob support, and optional conventional path helper
• Register builtin skill tools automatically: activate_skill and skill_search
• Add discoverable skill search with lightweight BM25 and structured system prompt rendering via Jinja
• Expose skill tool display names and descriptions for CLI/UI surfaces
• Add docs and an executable offline example

Validation

• uv run ruff check src/msgflux/runtime/skills.py src/msgflux/runtime/init.py src/msgflux/nn/modules/agent.py src/msgflux/tools/builtin/agent_skills.py src/msgflux/tools/builtin/init.py src/msgflux/init.py tests/test_agent_skills.py
• uv run pytest tests/test_agent_skills.py tests/nn/modules/test_agent.py tests/nn/modules/test_tool.py tests/generation/test_templates.py -q
• uv run python examples/agent_skills_demo.py
• uv run mkdocs build --strict