Refactor subagent runtime types and target validation

[mpscholten]

Summary

• extract SubagentRuntime, SubagentSession, and SubagentStoreRoot into an internal Types module
• extract persisted and active target validation into an internal Target module
• preserve the exact Agent.CLI.Subagents.Runtime public API
• keep implementation modules internal through Cabal other-modules
• regenerate package.nix

This is the first conservative Runtime split; backend and persistence orchestration remain together to avoid introducing dependency cycles in the same change.

Validation

• cabal build --offline -j1 --builddir=dist-pr5 agent-cli:lib:agent-cli
• SubagentStore-focused tests: 16 examples, 0 failures
• git diff --check
• regenerated package.nix matches cabal2nix .