feat: add skills system with built-in templates and per-agent custom skills

[Abhishek-Khanna24]

Summary

• Adds a Skills layer on top of the existing tools system — reusable, pre-packaged capabilities configurable entirely from YAML
• Ships three built-in templates: calculator (safe AST-based math, no eval), web_search (stub + Brave-ready), http_request (GET/POST via stdlib urllib)
• BaseSkill(BaseTool) integrates transparently with both ADK and LangGraph engine adapters — no adapter changes needed
• enabled: false flag lets users disable individual skills without removing the config entry
• skills_demo_agent demonstrates all built-ins plus an example_custom_skill.py copy-paste template for user-defined skills

How to use

skills:
  - id: math
    template: calculator            # built-in, zero Python required

  - id: search
    template: web_search
    enabled: false                  # disable without deleting
    config:
      provider: brave
      api_key_env: BRAVE_API_KEY

  - id: my_skill
    import: src.all_agents.my_agent.skills.my_skill.MySkill   # custom

Test plan

• 44 unit tests covering all skill templates and ToolManager integration (tests/unit/agent_framework/test_skills/)
• Full unit test suite passing — 222 passed, 13 skipped, 0 failures
• Start server and open AgentShip Studio → select skills_demo_agent
• Ask "what is 42 * 17?" — CalculatorSkill returns 714
• Ask "fetch https://httpbin.org/get" — HttpRequestSkill returns response JSON
• Ask "search for python tips" — WebSearchSkill returns stub message with setup instructions

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