feat: tag subcommand for adding tags and listing by tag

test/tag.bats

@@ -0,0 +1,64 @@
+#!/usr/bin/env bats
+
+load test_helper
+
+@test "tag adds a single tag to an alarm" {
+  run_nag at "tomorrow 3pm" "test alarm"
+  run_nag tag 1 work
+  [ "${status}" -eq 0 ]
+  local _tags
+  _tags="$(cut -f2 "${NAG_DIR}/alarms")"
+  [ "${_tags}" = "work" ]
+}
+
+@test "tag adds multiple tags to an alarm" {
+  run_nag at "tomorrow 3pm" "test alarm"
+  run_nag tag 1 work meetings
+  [ "${status}" -eq 0 ]
+  local _tags
+  _tags="$(cut -f2 "${NAG_DIR}/alarms")"
+  [ "${_tags}" = "work,meetings" ]
+}
+
+@test "tag merges with existing tags without duplicates" {
+  run_nag at "tomorrow 3pm" "test alarm"
+  run_nag tag 1 work
+  run_nag tag 1 meetings work
+  [ "${status}" -eq 0 ]
+  local _tags
+  _tags="$(cut -f2 "${NAG_DIR}/alarms")"
+  [ "${_tags}" = "work,meetings" ]
+}
+
+@test "tag rejects pure integer tag names" {
+  run_nag at "tomorrow 3pm" "test alarm"
+  run_nag tag 1 123
+  [ "${status}" -eq 1 ]
+}
+
+@test "tag with nonexistent ID fails" {
+  run_nag tag 99 work
+  [ "${status}" -eq 1 ]
+}
+
+@test "tag without arguments fails" {
+  run_nag tag
+  [ "${status}" -eq 1 ]
+}
+
+@test "tag with non-numeric arg lists matching alarms" {
+  run_nag at "tomorrow 3pm" "work task"
+  run_nag tag 1 work
+  run_nag at "tomorrow 4pm" "personal task"
+  run_nag tag work
+  [ "${status}" -eq 0 ]
+  [[ "${output}" =~ "work task" ]]
+  [[ ! "${output}" =~ "personal task" ]]
+}
+
+@test "tag list shows no matches message" {
+  run_nag at "tomorrow 3pm" "test alarm"
+  run_nag tag nonexistent
+  [ "${status}" -eq 0 ]
+  [[ "${output}" =~ "No alarms tagged" ]]
+}