docs: add GOOSE_SHELL env var

documentation/docs/guides/environment-variables.md

@@ -263,6 +263,7 @@ These variables control how goose manages conversation sessions and context.
 | `GOOSE_CONTEXT_STRATEGY` | Controls how goose handles context limit exceeded situations | "summarize", "truncate", "clear", "prompt" | "prompt" (interactive), "summarize" (headless) |
 | `GOOSE_MAX_TURNS` | [Maximum number of turns](/docs/guides/sessions/smart-context-management#maximum-turns) allowed without user input | Integer (e.g., 10, 50, 100) | 1000 |
 | `GOOSE_SUBAGENT_MAX_TURNS` | Sets the maximum turns allowed for a [subagent](/docs/guides/subagents) to complete before timeout. Can be overridden by [`settings.max_turns`](/docs/guides/recipes/recipe-reference#settings) in recipes or subagent tool calls. | Integer (e.g., 25) | 25 |
+| `GOOSE_MAX_BACKGROUND_TASKS` | Sets the maximum number of concurrent background [subagent](/docs/guides/subagents) tasks goose can run at once | Integer (e.g., 1, 5, 10) | 5 |
 | `CONTEXT_FILE_NAMES` | Specifies custom filenames for [hint/context files](/docs/guides/context-engineering/using-goosehints#custom-context-files) | JSON array of strings (e.g., `["CLAUDE.md", ".goosehints"]`) | `[".goosehints"]` |
 | `GOOSE_DISABLE_SESSION_NAMING` | Disables automatic AI-generated session naming; avoids the background model call and keeps the default "CLI Session" (goose CLI) or "New Chat" (goose Desktop) | "1", "true" (case-insensitive) to enable | false |
 | `GOOSE_PROMPT_EDITOR` | [External editor](/docs/guides/goose-cli-commands#external-editor-mode) to use for composing prompts instead of CLI input | Editor command (e.g., "vim", "code --wait") | Unset (uses CLI input) |
@@ -383,7 +384,8 @@ These variables control how goose handles [tool execution](/docs/guides/goose-pe
 | `GOOSE_CLI_MIN_PRIORITY` | Controls verbosity of [tool output](/docs/guides/managing-tools/adjust-tool-output) | Float between 0.0 and 1.0 | 0.0 |
 | `GOOSE_CLI_TOOL_PARAMS_TRUNCATION_MAX_LENGTH` | Maximum length for tool parameter values before truncation in CLI output (not in debug mode) | Integer | 40 |
 | `GOOSE_DEBUG` | Enables debug mode to show full tool parameters without truncation. Can also be toggled during a session using the `/r` [slash command](/docs/guides/goose-cli-commands#slash-commands) | "1", "true" (case-insensitive) to enable | false |
-| `GOOSE_SEARCH_PATHS` | Additional directories to search for executables when running extensions | JSON array of paths (e.g., `["/usr/local/bin", "~/custom/bin"]`) | System PATH only | No |
+| `GOOSE_SEARCH_PATHS` | Prepends additional directories to PATH for extension commands | JSON array of paths (for example, `["/usr/local/bin", "~/custom/bin"]`) | System PATH only |
+| `GOOSE_SHELL` | Overrides the shell used for Developer extension shell commands | Shell executable path or name (for example, `/bin/zsh`, `pwsh`, `C:\cygwin64\bin\bash.exe`) | Unix: `/bin/bash` if present, otherwise `$SHELL`, otherwise `sh`. Windows: `cmd` |
 
 **Examples**
 
@@ -397,9 +399,15 @@ export GOOSE_CLI_TOOL_PARAMS_MAX_LENGTH=100  # Show up to 100 characters for too
 
 # Add custom tool directories for extensions
 export GOOSE_SEARCH_PATHS='["/usr/local/bin", "~/custom/tools", "/opt/homebrew/bin"]'
+
+# Use zsh for Developer extension shell commands
+export GOOSE_SHELL=/bin/zsh
 ```
 
-These paths are prepended to the system PATH when extensions execute commands, ensuring your custom tools are found without modifying your global PATH.
+```bat
+REM Windows: use a POSIX-like shell instead of cmd.exe
+set GOOSE_SHELL=C:\cygwin64\bin\bash.exe
+```
 
 ### Enhanced Code Editing