fix: verify zsh exists before using it for hook execution (#544)

The `forceZsh` option on Linux/macOS would use a hardcoded zshPath
without checking if zsh actually exists on the system. This caused
hook commands to fail silently with exit code 127 on systems without
zsh installed.

Changes:
- Always verify zsh exists via findZshPath() before using it
- Fall back to bash -lc if zsh not found (preserves login shell PATH)
- Fall through to spawn with shell:true if neither found

The bash fallback ensures user PATH from .profile/.bashrc is available,
which is important for hooks that depend on custom tool locations.

Tested with opencode v1.1.3 - PreToolUse hooks now execute correctly
on systems without zsh.

Co-authored-by: Anas Viber <ananas-viber@users.noreply.github.com>

src/shared/command-executor.ts

@@ -5,16 +5,17 @@ import { existsSync } from "fs"
 import { homedir } from "os"
 
 const DEFAULT_ZSH_PATHS = ["/bin/zsh", "/usr/bin/zsh", "/usr/local/bin/zsh"]
+const DEFAULT_BASH_PATHS = ["/bin/bash", "/usr/bin/bash", "/usr/local/bin/bash"]
 
 function getHomeDir(): string {
   return process.env.HOME || process.env.USERPROFILE || homedir()
 }
 
-function findZshPath(customZshPath?: string): string | null {
-  if (customZshPath && existsSync(customZshPath)) {
-    return customZshPath
+function findShellPath(defaultPaths: string[], customPath?: string): string | null {
+  if (customPath && existsSync(customPath)) {
+    return customPath
   }
-  for (const path of DEFAULT_ZSH_PATHS) {
+  for (const path of defaultPaths) {
     if (existsSync(path)) {
       return path
     }
@@ -22,6 +23,14 @@ function findZshPath(customZshPath?: string): string | null {
   return null
 }
 
+function findZshPath(customZshPath?: string): string | null {
+  return findShellPath(DEFAULT_ZSH_PATHS, customZshPath)
+}
+
+function findBashPath(): string | null {
+  return findShellPath(DEFAULT_BASH_PATHS)
+}
+
 const execAsync = promisify(exec)
 
 export interface CommandResult {
@@ -55,10 +64,18 @@ export async function executeHookCommand(
   let finalCommand = expandedCommand
 
   if (options?.forceZsh) {
-    const zshPath = options.zshPath || findZshPath()
-    if (zshPath) {
+    // Always verify shell exists before using it
+    const zshPath = findZshPath(options.zshPath)
     const escapedCommand = expandedCommand.replace(/'/g, "'\\''")
+    if (zshPath) {
       finalCommand = `${zshPath} -lc '${escapedCommand}'`
+    } else {
+      // Fall back to bash login shell to preserve PATH from user profile
+      const bashPath = findBashPath()
+      if (bashPath) {
+        finalCommand = `${bashPath} -lc '${escapedCommand}'`
+      }
+      // If neither zsh nor bash found, fall through to spawn with shell: true
     }
   }