feat(hooks): tomd-preread PreToolUse(Read) auto-convert hook

POSIX sh hook (50 LOC) that intercepts Read on .docx/.doc/.xlsx/.pptx/.csv
and auto-redirects Claude to a cached markdown conversion via the tomd
primitive.

- jq absence → exit 0 (graceful degrade, matches assemble-validate style)
- tomd primitive missing → exit 0 (don't block)
- cache dir via KEISEI_TOMD_CACHE env (default /tmp/keisei-tomd-cache)
- cache key = basename + mtime, portable stat for macOS/Linux
- exit 2 with [tomd-preread] stderr message on successful conversion
- conversion failure → exit 0 (let Claude try original, fail naturally)

Not wired into settings-snippet.json yet — follow-up commit adds the
PreToolUse(Read) entry alongside install.sh hooks-copy loop extension.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

hooks/tomd-preread.sh

@@ -0,0 +1,50 @@
+#!/bin/sh
+# PreToolUse(Read) — auto-convert non-native formats to markdown and redirect
+# Claude to read the converted file instead of the opaque binary.
+#
+# Exit 0 = allow (passthrough). Exit 2 = block with stderr message (Claude
+# reads the stderr text and switches to the converted path).
+#
+# Stdin: JSON with tool_input.file_path.
+
+# Silent fall-through if jq is absent; otherwise `set -eu` would abort and
+# Claude Code would refuse Read system-wide.
+command -v jq >/dev/null 2>&1 || exit 0
+
+set -eu
+
+TOMD="$HOME/.claude/agents/_primitives/tomd.sh"
+CACHE_DIR="${KEISEI_TOMD_CACHE:-/tmp/keisei-tomd-cache}"
+
+FILE=$(jq -r '.tool_input.file_path // empty')
+[ -n "$FILE" ] || exit 0
+[ -f "$FILE" ] || exit 0
+
+# Extension whitelist — only these formats trigger conversion.
+EXT=$(printf '%s' "${FILE##*.}" | tr '[:upper:]' '[:lower:]')
+case "$EXT" in
+    docx|doc|xlsx|pptx|csv) ;;
+    *) exit 0 ;;
+esac
+
+# tomd primitive must be installed; if absent, don't block the Read.
+[ -x "$TOMD" ] || exit 0
+
+mkdir -p "$CACHE_DIR"
+
+# Cache key: basename + mtime. Portable stat for macOS + Linux.
+BASENAME=$(basename "$FILE")
+MTIME=$(stat -f %m "$FILE" 2>/dev/null || stat -c %Y "$FILE" 2>/dev/null || echo 0)
+MD_FILE="$CACHE_DIR/${BASENAME%.*}-${MTIME}.md"
+
+if [ ! -s "$MD_FILE" ]; then
+    "$TOMD" "$FILE" > "$MD_FILE" 2>/dev/null || true
+fi
+
+if [ -s "$MD_FILE" ]; then
+    echo "[tomd-preread] Auto-converted to markdown: $MD_FILE. Use Read on $MD_FILE instead of $FILE." >&2
+    exit 2
+fi
+
+# Conversion failed or produced empty output — degrade gracefully.
+exit 0