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/session-start-context.sh

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+# SessionStart context: print branch, dirty file count, and last commit.
+
+cd "$CLAUDE_PROJECT_DIR" || exit 0
+
+branch=$(git branch --show-current 2> /dev/null)
+dirty=$(git status --short 2> /dev/null | wc -l | tr -d ' ')
+last=$(git log -1 --oneline 2> /dev/null)
+
+echo "=== Dotfiles session context ==="
+echo "Branch : ${branch:-unknown}"
+echo "Dirty  : ${dirty} file(s)"
+echo "Last   : ${last}"
+
+exit 0