Add documentation for GOOSE_TERMINAL environment variable

[blackgirlbytes]

Problem
The GOOSE_TERMINAL environment variable feature was added in PR #3911, but there's no documentation explaining how users can leverage this feature.

Solution
Add documentation that explains:

• What the GOOSE_TERMINAL environment variable is
• When it gets set (all command executions by Goose)
• How users can use it to customize their shell environment
• Practical examples of use cases

Suggested Documentation Locations

• User guide/configuration section
• Environment variables reference
• Developer extension documentation

Example Use Cases to Document

1. Custom aliases for Goose sessions:

   if [[ -n "$GOOSE_TERMINAL" ]]; then
     alias find="echo 'Use rg (ripgrep) instead of find for better performance'"
     alias ls="ls --color=auto"
   fi

2. Goose-specific PATH modifications:

   if [[ -n "$GOOSE_TERMINAL" ]]; then
     export PATH="/usr/local/goose-tools:$PATH"
   fi

3. Custom prompt indicators:

   if [[ -n "$GOOSE_TERMINAL" ]]; then
     export PS1="[GOOSE] $PS1"
   fi

Acceptance Criteria

• Documentation explains what GOOSE_TERMINAL is and when it's set
• Includes practical examples of how to use it
• Documents best practices and common use cases
• Links to related PR feat: Add GOOSE_TERMINAL env variable to spawned terminals #3911 for context
• Available in appropriate sections of the documentation

Related

• Implements feature from PR feat: Add GOOSE_TERMINAL env variable to spawned terminals #3911
• Addresses issue Add GOOSE_TERMINAL environment variable to spawned terminals under the goose agent for custom profile changes #3827

[blackgirlbytes]

goose made this issue. i dont want it.