feat(config): add `subagent_tools` to complement `primary_tools`

[spoons-and-mirrors]

What does this PR do?

feat: Adds subagent_tools to the experimental config to allow specifying tools that should only be available to subagents, complementing primary_tools

refactor: moved the primary tools filtering to the tool registry for consistency (as subagent tools needs to be constructed at the registry level)