Add composable multi-language linting and formatting system

PostToolUse hook auto-formats files after Edit/Write/MultiEdit with
git-blob checkpoints for safe revert. Pre-commit hook for staged files.
Canonical formatter scripts for py, sh, ts, sql, json (+ symlinks for
js, yaml, md). Install and setup scripts for project opt-in.

Includes best-practices/linting.md, HOOKS.md docs, README.md, and
session log.

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

formatters/json

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+# Formatter: JSON/YAML/Markdown — prettier
+# Contract: $1 = absolute file path, format in place, lint, exit 0 if clean / 1 if errors
+# Missing tools: exit 0 silently
+
+FILE="$1"
+[ -z "$FILE" ] && exit 0
+[ -f "$FILE" ] || exit 0
+
+command -v prettier >/dev/null 2>&1 || exit 0
+
+# Format in place (prettier auto-detects parser from extension)
+ERRORS=$(prettier --write "$FILE" 2>&1)
+if [ $? -ne 0 ]; then
+    echo "$ERRORS" >&2
+    exit 1
+fi
+
+exit 0