[Agents] Ensure that subagents can support individual policy

Currently policies are inherited by the primary agent. Add the necessary logic needed to support policy for individual subagents (via config, etc.)

The goal here is that the main agent and subagent(s) should not have to share tools. Today, the subagent can have a subset of tools available to the model.

We need to have the flexibility where the subagent can be provided X tools that are then hidden from the main agent.

This includes built-in tools and MCP server tools as well. Extensions can provide MCP servers/tools. We should ensure that there is flexibility for extension authors to create subagents that limit MCP tools to subagents and do not expose them to the main agent.

Some considerations:
- How will policy be configured within the yaml frontmatter
- How does this work with extensions policy (should talk to @chrstnb) to ensure compatibility w/ general extension policy goals, etc