Pipe SessionStart hook printf through cat to absorb EPIPE on Windows

On Windows + Git Bash, the SessionStart hook prints a confusing
diagnostic at every startup ("printf: write error: Permission denied")
when Claude Code closes the hook's stdout pipe before the printf has
finished writing. The hook still runs to completion and context still
gets injected, but the diagnostic surfaces every session because
Git Bash's printf reports EPIPE as "Permission denied" (not "Broken
pipe" like Linux) and our `set -euo pipefail` lets that error escape.

Piping each printf through `cat` makes the external cat process the
recipient of any SIGPIPE / EPIPE. cat's failure does not propagate to
the parent bash under pipefail because cat is the last command in the
pipeline and exits cleanly when the pipe stays open long enough to
hold the data. On macOS/Linux the cat passthrough is transparent (no
behavior change, no measurable cost).

Verified:
- Existing tests/hooks/test-session-start.sh: 7/7 pass on macOS
- Manual run on Windows 11 + Git Bash 5.2 + Node 22 produces valid JSON,
  clean stderr, and exit 0
- JSON output is byte-identical to the unpatched hook

Reported by @silvertakana in #1612, attribution preserved in the
Co-authored-by trailer below — this is the same fix shape the original
PR proposed.

Co-authored-by: silvertakana <silvertakana@users.noreply.github.com>
Closes #1612.

hooks/session-start

@@ -37,13 +37,13 @@ session_context="<EXTREMELY_IMPORTANT>\nYou have superpowers.\n\n**Below is the
 # See: https://github.com/obra/superpowers/issues/571
 if [ -n "${CURSOR_PLUGIN_ROOT:-}" ]; then
   # Cursor sets CURSOR_PLUGIN_ROOT (may also set CLAUDE_PLUGIN_ROOT)
-  printf '{\n  "additional_context": "%s"\n}\n' "$session_context"
+  printf '{\n  "additional_context": "%s"\n}\n' "$session_context" | cat
 elif [ -n "${CLAUDE_PLUGIN_ROOT:-}" ] && [ -z "${COPILOT_CLI:-}" ]; then
   # Claude Code sets CLAUDE_PLUGIN_ROOT without COPILOT_CLI
-  printf '{\n  "hookSpecificOutput": {\n    "hookEventName": "SessionStart",\n    "additionalContext": "%s"\n  }\n}\n' "$session_context"
+  printf '{\n  "hookSpecificOutput": {\n    "hookEventName": "SessionStart",\n    "additionalContext": "%s"\n  }\n}\n' "$session_context" | cat
 else
   # Copilot CLI (sets COPILOT_CLI=1) or unknown platform — SDK standard format
-  printf '{\n  "additionalContext": "%s"\n}\n' "$session_context"
+  printf '{\n  "additionalContext": "%s"\n}\n' "$session_context" | cat
 fi
 
 exit 0

hooks/session-start-codex

@@ -21,6 +21,6 @@ escape_for_json() {
 using_superpowers_escaped=$(escape_for_json "$using_superpowers_content")
 session_context="<EXTREMELY_IMPORTANT>\nYou have superpowers.\n\n**Below is the full content of your 'superpowers:using-superpowers' skill - your introduction to using skills. For all other skills, follow the Codex skill-loading instructions in that skill:**\n\n${using_superpowers_escaped}\n</EXTREMELY_IMPORTANT>"
 
-printf '{\n  "hookSpecificOutput": {\n    "hookEventName": "SessionStart",\n    "additionalContext": "%s"\n  }\n}\n' "$session_context"
+printf '{\n  "hookSpecificOutput": {\n    "hookEventName": "SessionStart",\n    "additionalContext": "%s"\n  }\n}\n' "$session_context" | cat
 
 exit 0