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.md

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+# Hooks
+
+Claude Code hooks that run automatically in response to events. Source of truth lives here; symlinks point from `~/.claude/hooks/` back to this directory.
+
+## Available Hooks
+
+| Hook | Event | File |
+|------|-------|------|
+| Pre-compact backup | PreCompact (auto + manual) | `hooks/pre-compact-backup.sh` |
+
+## Pre-compact Backup
+
+Saves a copy of the session transcript before context compaction so no conversation history is lost. Backs up to `~/.claude/transcript-backups/` with timestamped filenames. Auto-prunes backups older than 30 days.
+
+**Triggers:** Both auto-compaction (context window full) and manual (`/compact` command).
+
+**Requires:** `python3` (for JSON parsing from stdin).
+
+## Installation
+
+1. Symlink each hook into `~/.claude/hooks/`:
+   ```bash
+   mkdir -p ~/.claude/hooks
+   ln -sf "$(pwd)/hooks/pre-compact-backup.sh" ~/.claude/hooks/pre-compact-backup.sh
+   ```
+
+2. Add the hook configuration to `~/.claude/settings.json`:
+   ```json
+   {
+     "hooks": {
+       "PreCompact": [
+         {
+           "matcher": "auto",
+           "hooks": [{ "type": "command", "command": "~/.claude/hooks/pre-compact-backup.sh", "timeout": 15 }]
+         },
+         {
+           "matcher": "manual",
+           "hooks": [{ "type": "command", "command": "~/.claude/hooks/pre-compact-backup.sh", "timeout": 15 }]
+         }
+       ]
+     }
+   }
+   ```
+
+## Adding New Hooks
+
+1. Create the script in `hooks/`
+2. Add an entry to this file
+3. Symlink into `~/.claude/hooks/`
+4. Add the matcher config to `~/.claude/settings.json`