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/log-failures.sh

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+# PostToolUseFailure logger: append tool failures to a local log file.
+
+log_file="$CLAUDE_PROJECT_DIR/.claude/hook-failures.log"
+
+entry=$(jq -c '{
+  time: (now | strftime("%Y-%m-%dT%H:%M:%SZ")),
+  tool: .tool_name,
+  error: .error
+}')
+
+echo "$entry" >> "$log_file"
+
+exit 0