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>

hooks/pre-commit-lint.sh

@@ -0,0 +1,54 @@
+#!/usr/bin/env bash
+# Git pre-commit hook: format and lint staged files using project formatters.
+# Reuses the same formatter scripts from formatters/.
+# Exits non-zero if lint errors remain after auto-fix (blocks commit).
+
+# Find formatters/ directory relative to repo root
+REPO_ROOT="$(git rev-parse --show-toplevel 2>/dev/null)"
+[ -z "$REPO_ROOT" ] && exit 0
+
+FORMATTERS_DIR="$REPO_ROOT/formatters"
+[ -d "$FORMATTERS_DIR" ] || exit 0
+
+HAD_ERRORS=0
+ERROR_OUTPUT=""
+
+# Iterate staged files (only Added/Modified, not Deleted)
+while IFS= read -r FILE; do
+    [ -z "$FILE" ] && continue
+
+    ABS_FILE="$REPO_ROOT/$FILE"
+    [ -f "$ABS_FILE" ] || continue
+
+    # Extract extension (normalise yml → yaml)
+    EXT="${FILE##*.}"
+    case "$EXT" in
+        yml) EXT="yaml" ;;
+    esac
+    [ -z "$EXT" ] || [ "$EXT" = "$FILE" ] && continue
+
+    # Check for formatter
+    FORMATTER="$FORMATTERS_DIR/$EXT"
+    [ -x "$FORMATTER" ] || continue
+
+    # Run formatter
+    ERRORS=$("$FORMATTER" "$ABS_FILE" 2>&1)
+    EXIT_CODE=$?
+
+    # Re-stage the file (formatter may have modified it)
+    git add "$ABS_FILE"
+
+    if [ $EXIT_CODE -ne 0 ]; then
+        HAD_ERRORS=1
+        ERROR_OUTPUT="${ERROR_OUTPUT}${FILE}:\n${ERRORS}\n\n"
+    fi
+done < <(git diff --cached --name-only --diff-filter=AM)
+
+if [ $HAD_ERRORS -ne 0 ]; then
+    echo "Pre-commit lint errors — commit blocked:" >&2
+    echo -e "$ERROR_OUTPUT" >&2
+    echo "Fix the errors above and try again." >&2
+    exit 1
+fi
+
+exit 0