SessionStart hook fails on Windows/Cygwin: backslash path not translated for /bin/bash

Bug Description

Plugin hooks with SessionStart event fail on Windows when Cygwin bash is the configured shell. The ${CLAUDE_PLUGIN_ROOT} variable is expanded to a Windows-style path (with backslashes), but the hook command is executed via Cygwin's /bin/bash, which strips the backslashes and produces an invalid path.

Steps to Reproduce

1. Install Claude Code on Windows with Cygwin as the shell
2. Install any plugin with a SessionStart hook (e.g., superpowers, explanatory-output-style)
3. Start a new session

Expected Behavior

Hooks execute successfully. The plugin root path should be translated to a Cygwin-compatible format (e.g., /c/Users/sho/.claude/plugins/cache/...) before being passed to /bin/bash.

Actual Behavior

Hooks fail with "No such file or directory". From the debug log:

Hook SessionStart:startup (SessionStart) error:
/bin/bash: C:Userssho.claudepluginscacheclaude-plugins-officialsuperpowers4.3.0/hooks/session-start.sh: No such file or directory

The Windows path C:\Users\sho\.claude\plugins\cache\claude-plugins-official\superpowers\4.3.0 has its backslashes stripped by Cygwin bash, resulting in the mangled path C:Userssho.claudeplugins....

Root Cause

When Claude Code constructs the hook command, it substitutes ${CLAUDE_PLUGIN_ROOT} with the Windows-native path (backslashes). It then invokes the command via Cygwin's /bin/bash. Cygwin bash interprets backslashes as escape characters rather than path separators, producing an invalid path.

Running the same scripts manually from a Cygwin terminal works fine because the paths are already in Unix format.

Environment

• OS: Windows 11 Pro 10.0.26200
• Shell: Cygwin bash (/bin/bash)
• Affected plugins: superpowers@claude-plugins-official (v4.3.0), explanatory-output-style@claude-plugins-official

Suggested Fix

When the detected shell is Cygwin bash (i.e., /bin/bash on Windows), convert CLAUDE_PLUGIN_ROOT and the command path to POSIX format before execution. Cygwin provides cygpath for this:

cygpath -u "C:\Users\sho\.claude\plugins\cache\..."
# → /c/Users/sho/.claude/plugins/cache/...

Alternatively, normalize all plugin paths to use forward slashes before substitution, which works in both Cygwin and Git Bash on Windows.