nushell: load session environment at runtime

[LisaScheers]

What changed

• Generate a Nushell env.nu loader when a system or Home Manager session
  environment needs to be applied.
• At shell startup, source the OS-generated setEnvironment script when Home Manager is used as a NixOS or nix-darwin module, followed by hm-session-vars.sh.
• Capture only variables changed by those scripts and load them into Nushell, preserving PATH as a list<string>.
• Keep the existing source guards and load the generated environment before the user's envFile/extraEnv, so inherited or user-specific values keep their expected precedence.
• Document the new env.nu ownership and migration path in the 26.11 release
  notes.

Why

Nushell does not source the POSIX environment scripts used by Bash and Zsh. As a result, a directly launched Nushell login shell can miss the Nix profile PATH, Home Manager session variables, and expansions involving runtime values such as $HOME and $USER.

Evaluating these scripts at build time captures sandbox values instead of the user's login environment. This implementation evaluates them at shell startup and transfers the resulting environment through a NUL-delimited representation, which also handles values containing newlines.

This builds on the investigation and experiments in #6941 while avoiding its sandbox-environment limitation.

User impact

Nushell can be launched directly instead of through Zsh or Bash while receiving the system and Home Manager environment. home.sessionVariables, home.sessionPath, POSIX default/alternate expansions, and NixOS/nix-darwin profile paths are applied consistently.

Fixes #4313
Fixes #4918
Fixes #6507
Addresses #6568
Fixes #6901

Validation

• Added a runtime test that launches Nushell from an intentionally minimal environment and verifies OS variables, $HOME/$USER expansion, POSIX default/alternate expansion, home.sessionPath, list-form PATH, and user env.nu precedence.
• Ran the full test suite with nix build .#test-all.
• Built all 13 Nushell-focused Home Manager module tests on aarch64-darwin.
• Evaluated the new test derivation for x86_64-linux.
• Rendered the HTML manual with nix build .#docs-html.
• Ran the repository formatter, the 80-column check for added lines, and
  git diff --check.

[iniw]

Note that this PR still doesn't you to seamlessly set nushell as the login shell on macOS if you want the config to be stored in XDG directories. If XDG_CONFIG_HOME is not set during startup, nushell will look for the config file in ~/Library/Application Support/nushell, which is not where home manager has placed it if you have the XDG-related options enabled. I'm not sure what the best way to fix it is 🤷

[LisaScheers]

the real solution is to let the xdg module write a plist with something like

<!-- ~/Library/LaunchAgents/setenv.XDG_CONFIG_HOME.plist -->
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
  <plist version="1.0">
  <dict>
  <key>Label</key>
  <string>setenv.XDG_CONFIG_HOME</string>
  <key>ProgramArguments</key>
  <array>
    <string>/bin/launchctl</string>
    <string>setenv</string>
    <string>XDG_CONFIG_HOME</string>
    <string>{USER_HOME}/.config</string>
  </array>
  <key>RunAtLoad</key>
  <true/>
</dict>
</plist>

to ~/Library/LaunchAgents/setenv.XDG_CONFIG_HOME.plist

this would set it for all applications