Add reference env and dotfile examples

[codeforester]

Summary

• Add inactive reference examples for direnv, asdf, chezmoi, and dotbot under examples/tooling/env-dotfiles/.
• Document why these examples stay out of root-level active files by default.
• Add contract and validation guards that prevent root .envrc, .tool-versions, or dotbot installer files from becoming accidental baseline behavior.

Fixes #179.

Validation

• ./tests/validate.sh
• git diff --check