feat(config): discover treehouse.toml in ancestor directories

[rudingma]

Summary

• discover the nearest treehouse.toml from the repository root upward
• stop before $HOME or the filesystem root
• resolve relative root values from the selected config file's directory
• preserve repo-root precedence, user-level hooks, global config, and global prune behavior
• document ancestor configuration and make config-committing E2E fixtures independent of global ignore rules

Why

Pool placement is operator and machine policy, but repository-root-only discovery requires duplicating the same untracked configuration across every repository in a context. Ancestor discovery lets a context directory such as ~/Workspace/personal own that policy while repository-root files remain available as explicit overrides.

The implementation stays behind the existing config.Load(repoRoot) interface, so all repo-scoped commands receive the behavior consistently without changing their callers.

Compatibility

• existing repository-root files remain the nearest file and keep precedence
• only one repo-scoped file is selected; configs are not cascaded or merged
• $HOME/treehouse.toml is not treated as a second global config
• malformed nearest configs fail instead of silently falling back
• hooks in every repo-scoped config remain ignored
• treehouse init, user-level config, and prune --all retain their existing roles

Validation

• make lint
• make test
• GOOS=windows GOARCH=amd64 go build ./...
• git diff --check

Closes #75