feat(agentkit): centralize generated app infrastructure

[yaozheng-fang]

Summary

• add a public veadk.integrations.agentkit application factory and runner
• move health, topology, Web UI, memory, and Feishu lifecycle handling out of generated projects
• reduce generated app.py to a minimal 11-line entry point
• add a complete generated project example and update documentation

Compatibility

• existing generated projects continue to run unchanged
• newly generated projects require veadk-python>=1.0.5

Validation

• 40 targeted tests passed
• Ruff, Ruff format, Pyright, pre-commit, and MDX generation passed
• deployed the generated example to an AgentKit Runtime and verified health, app discovery, topology, session creation, and SSE conversation endpoints