Complete XDG-aware RubyGems lookup and executable PATH setup across shells

[Copilot]

Mantle already set XDG-oriented RubyGems storage, but not the corresponding lookup and executable discovery contract. This change makes Ruby gem installs predictable across shell initialization by aligning GEM_HOME, GEM_PATH, GEM_SPEC_CACHE, and ${GEM_HOME}/bin behavior in Bash/Zsh/Fish without clobbering explicit user overrides.

• RubyGems environment contract

  • Default GEM_HOME to "$XDG_DATA_HOME/gem" when unset.
  • Keep GEM_SPEC_CACHE under "$XDG_CACHE_HOME/gem/specs".
  • Derive GEM_PATH from the active gem env path rather than forcing GEM_PATH="$GEM_HOME", so system/default gem paths remain visible.
  • Use Ruby’s platform path separator when parsing and exporting GEM_PATH.

• Executable discovery

  • Project ${GEM_HOME}/bin into Mantle’s deterministic PATH construction for Bash/Zsh and Fish.
  • Add the gem bin path even before the directory exists so executables installed later in the same shell become immediately discoverable.
  • Normalize trailing slashes and avoid duplicate logical PATH entries across repeated initialization.

• Override and safety behavior

  • Preserve explicit GEM_HOME, GEM_PATH, and GEM_SPEC_CACHE.
  • Warn instead of guessing when GEM_HOME is relative; in that case Mantle does not derive GEM_PATH or project a gem bin path.
  • Surface ambiguous Ruby-manager situations by warning when Mantle cannot safely determine a RubyGems lookup path.

• Cross-shell alignment

  • Apply the same RubyGems policy to the POSIX/Bash/Zsh runtime path and the native Fish environment path.
  • Keep shell-specific implementation details consistent around normalization and deduplication.

• Documentation and hermetic coverage

  • Document the RubyGems/XDG contract, including interaction with system Ruby, Homebrew Ruby, asdf, rbenv, mise, and Bundler.
  • Add isolated integration coverage for default behavior, user overrides, re-sourcing, same-shell executable resolution, relative GEM_HOME, and live RubyGems path derivation when a real runtime is present.

export GEM_HOME="${XDG_DATA_HOME}/gem"
export GEM_SPEC_CACHE="${XDG_CACHE_HOME}/gem/specs"
export GEM_PATH="$(gem env path)"
export PATH="${GEM_HOME}/bin:${PATH}"

• Fixes feat(environment): complete XDG-aware Ruby gem configuration and executable PATH support #29