Fetch rootless etc core during bootstrap

[jasonmorganson]

Summary

• keep dotfiles as the one-command entrypoint while fetching the immutable etc v1.0.0 core
• install mise under ~/.local/bin with no sudo or /etc writes
• compose 00-etc.toml with the existing personal 10-50 fragments and one merged global lock
• require the canonical ~/.local/share/dotfiles checkout for durable reapplication
• update CI and the devcontainer for the rootless paths

Verification

• clean composed Ubuntu install and reinstall with no bootstrap drift
• Bash, Zsh, Fish, and Nushell activation/XDG checks
• five-platform install matrix: Ubuntu 22.04/24.04 and macOS 14/15/26
• amd64 and arm64 devcontainer image builds
• dirty and wrong-origin etc checkouts refused; clean wrong-ref checkout converges to the pinned SHA

Core PR: jasonmorganson/etc#1
Immutable release: https://github.com/jasonmorganson/etc/releases/tag/v1.0.0

[jasonmorganson]

Closing without merge. The personal dotfiles remain authoritative: mise stays at /usr/local/bin, XDG and shell activation stay in dotfiles, and jasonmorganson/etc remains a separate optional standalone baseline rather than a required runtime dependency.