feat: add placeholder main.py for BYO agent on project create

[notgitika]

Description

When a user creates a project with "Bring your own agent" (--type byo), the agent code directory (app/<AgentName>/) is left completely empty. The CLI output tells the user:

Copy your agent code to app/MyAgent/ before deploying.
Ensure main.py is the entrypoint file in that folder.

This is confusing — the user has to know to create main.py themselves. A better experience would be to vend a minimal placeholder main.py with a basic entrypoint stub so the directory isn't empty and the user has a starting point.

Acceptance Criteria

• When "Bring your own agent" is selected during agentcore create or agentcore add agent --type byo, a placeholder main.py is created in the agent's code directory
• The placeholder should contain a minimal working entrypoint (e.g., a simple handler function with a TODO comment)
• The placeholder should match the expected entrypoint contract so agentcore dev works out of the box

Additional Context

Current behavior screenshot from agentcore create with BYO agent:

Created:
  SampleAgents/
    app/MyAgent/  Agent code location (empty)
    agentcore/    Config and CDK project

The (empty) note highlights the gap — every other agent type gets template code vended, but BYO gets nothing.