feat!: add builtin templates

WIP: document, check for breaking changes, decide on version, etc.

templates/md

@@ -0,0 +1,42 @@
+#!/bin/sh
+set -eu
+type agenda >/dev/null
+
+if [ "${AGENDA_EXTENSION:-}" != "md" ]; then
+    exit
+fi
+
+# Capitalize the title.
+echo "$AGENDA_NAME $AGENDA_DATE" | awk '{print "# "toupper(substr($0,0,1))tolower(substr($0,2))}'
+echo
+
+if [ -n "${AGENDA_IS_BACKLOG:-}" ]; then
+    exit
+fi
+
+# Carry over open tasks from the last agenda markdown file.
+if [ -e "$AGENDA_LAST" ] && [ "${AGENDA_LAST#*.}" = "md" ]; then
+    while IFS= read -r REPLY; do
+        case ${state:-} in
+            "open_task")
+                case $REPLY in
+                    "- [ ] "*|"      "*)
+                        # Append indented lines to open tasks.
+                        echo "$REPLY"
+                        ;;
+                    *)
+                        unset state
+                        ;;
+                esac
+                ;;
+            *)
+                case $REPLY in
+                    "[ ] "*|"[@] "*|"[?] "*)
+                        state="open_task"
+                        echo "$REPLY"
+                        ;;
+                esac
+                ;;
+        esac
+    done < "$AGENDA_LAST"
+fi