Move pre-compact hook into repo with HOOKS.md documentation

Hook source of truth now in hooks/, symlinked from ~/.claude/hooks/.
HOOKS.md covers what each hook does and installation steps.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

hooks/pre-compact-backup.sh

@@ -0,0 +1,30 @@
+#!/usr/bin/env bash
+# Pre-compact hook: save a copy of the session transcript before compaction.
+# Claude Code pipes JSON to stdin with session_id, transcript_path, cwd, etc.
+
+set -euo pipefail
+
+# Parse stdin JSON for transcript path and session ID
+INPUT="$(cat)"
+TRANSCRIPT_PATH="$(echo "$INPUT" | python3 -c "import sys,json; print(json.load(sys.stdin).get('transcript_path',''))" 2>/dev/null)"
+SESSION_ID="$(echo "$INPUT" | python3 -c "import sys,json; print(json.load(sys.stdin).get('session_id',''))" 2>/dev/null)"
+TRIGGER="$(echo "$INPUT" | python3 -c "import sys,json; print(json.load(sys.stdin).get('trigger','unknown'))" 2>/dev/null)"
+
+if [ -z "$TRANSCRIPT_PATH" ] || [ ! -f "$TRANSCRIPT_PATH" ]; then
+    echo "pre-compact-backup: no transcript found, skipping" >&2
+    exit 0
+fi
+
+# Save to ~/.claude/transcript-backups/ with timestamp
+BACKUP_DIR="$HOME/.claude/transcript-backups"
+mkdir -p "$BACKUP_DIR"
+
+TIMESTAMP="$(date +%Y%m%d-%H%M%S)"
+BACKUP_FILE="${BACKUP_DIR}/${TIMESTAMP}-${SESSION_ID:0:8}-${TRIGGER}.jsonl"
+
+cp "$TRANSCRIPT_PATH" "$BACKUP_FILE"
+
+# Prune backups older than 30 days
+find "$BACKUP_DIR" -name "*.jsonl" -mtime +30 -delete 2>/dev/null || true
+
+echo "pre-compact-backup: saved $(wc -l < "$BACKUP_FILE") lines to ${BACKUP_FILE##*/}" >&2