refactor(claude): migrate hooks to external scripts and add new hooks

Replace inline command strings in settings.json with external scripts
in .claude/hooks/ for readability and maintainability. Consolidate
three PostToolUse formatters into one script and add markdown/yaml
formatting. Add new hooks: SessionStart context banner, Stop lint
gate, async Bats test runner, idle desktop notification, and
PostToolUseFailure logger.

.claude/hooks/async-bats.sh

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+# Async Bats runner: run matching test file when a script is edited.
+# Runs in background (async: true) — output appears on next turn.
+
+fp=$(jq -r '.tool_input.file_path // empty')
+[ -z "$fp" ] && exit 0
+
+name=$(basename "$fp")
+test_file="$CLAUDE_PROJECT_DIR/tests/${name}.bats"
+[ ! -f "$test_file" ] && exit 0
+
+echo "Running $test_file ..."
+"$CLAUDE_PROJECT_DIR/node_modules/.bin/bats" "$test_file"