Proposal: Configuration based tools

[adam-paterson]

We're seeing a lot of "Add Agent X" PRs and duplicate template copying logic. It would be good to refactor the tool checks and template downloads to be configuration-based.

We're also seeing a lot of mismatched templates for specific agents: All additional agent tools currently reference CLAUDE.md in the downloaded constitution.md, for example.

Creating an agents.yml or agents.toml would make it easier for people to add their favourite agents to the CLI.