feat: add AGENT=goose environment variable for cross-tool compatibility

[sw-square]

Summary

This PR adds a generic AGENT=goose environment variable that is set alongside GOOSE_TERMINAL=1 in all shell command executions.

Motivation

There's quite a bit of setup in my configuration files that I don't want AI agents to use. It's quite annoying having to check for each individual agent's environment variables:

# ~/.zshrc
if [[  $CLAUDECODE != 1 && $GOOSE_TERMINAL != 1 && $AGENT != amp ]] then
	# setup non-agent stuff that can take awhile
	PS1= # ... setup PS1
	bindkey # ... custom keybinds for terminal
	source /.../zsh-syntax-highlighting.zsh # syntax highlighting and autosuggestions
	source /.../zsh-autosuggestions.zsh
	# modify path to include interactive scripts I don't want agents to use
	path+=( ~/_bin )
fi

I noticed that amp uses AGENT=amp, and I figured that's a good environment variable name to standardize on. This PR adds that in.

Note that this complements the GOOSE_TERMINAL=1 environment variable; it might be nice to eventually remove GOOSE_TERMINAL if AGENT becomes standardized.

[Copilot]

Pull request overview

This PR adds a generic AGENT=goose environment variable to complement the existing GOOSE_TERMINAL=1 variable for better cross-tool compatibility. This enables users to write shell configuration that works across multiple AI agents without hardcoding agent-specific checks, following a pattern established by other agents like amp.

Changes:

• Added AGENT=goose environment variable to all shell command executions alongside GOOSE_TERMINAL=1
• Updated documentation to explain the new variable's purpose and provide usage examples
• Maintains backward compatibility with existing GOOSE_TERMINAL checks

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
documentation/docs/guides/environment-variables.md	Documents the new AGENT variable with clear examples and use cases
crates/goose/src/agents/retry.rs	Adds AGENT=goose to shell command execution for both Windows (cmd) and Unix (sh)
crates/goose-mcp/src/developer/shell.rs	Adds AGENT=goose to the shell command configuration
crates/goose-mcp/src/computercontroller/platform/windows.rs	Adds AGENT=goose to Windows PowerShell automation commands
crates/goose-mcp/src/computercontroller/mod.rs	Adds AGENT=goose to both PowerShell and Unix shell executions
crates/goose-cli/src/commands/update.rs	Adds AGENT=goose to the update script execution