feat(completion): make the engine the default behind --completion

The engine shipped opt-in behind --new-completion, with the old scripts
still on --completion. That split was never released, so flip it now
rather than carry a third flag through a deprecation later.

--completion serves the engine wrappers; the hand-written scripts move to
completion/legacy/ and stay reachable via --legacy-completion for a
release or two.

.goreleaser.yml needed to follow: it packages the static files from
completion/ into the deb/rpm/apk contents and the Homebrew cask, so
leaving it untouched would have shipped the engine to anyone running
`eval "$(task --completion zsh)"` and the old scripts to everyone
installing from a package. The paths it references are unchanged and now
resolve to the wrappers. Its archive glob is narrowed at the same time,
so the test harness under completion/tests/ stops being shipped in the
release archives.

completion/bash/task.bash

@@ -1,60 +1,94 @@
 # vim: set tabstop=2 shiftwidth=2 expandtab:
+#
+# Thin wrapper around `task __complete`: all suggestion logic lives in the Go engine.
 
-_GO_TASK_COMPLETION_LIST_OPTION='--list-all'
 TASK_CMD="${TASK_EXE:-task}"
 
-function _task()
-{
-  local cur prev words cword
-  _init_completion -n : || return
+# `=` stays inside the current word (see `_init_completion -n =:`), so an inline
+# `--flag=` prefix must be stripped before _filedir and re-applied after.
+_task_filedir() {
+  local fpfx="" savecur="$cur"
+  if [[ "$cur" == -*=* ]]; then
+    fpfx="${cur%%=*}="
+    cur="${cur#*=}"
+  fi
+  _filedir ${1:+"$1"}
+  cur="$savecur"
+  if [[ -n "$fpfx" ]]; then
+    COMPREPLY=( ${COMPREPLY[@]+"${COMPREPLY[@]/#/$fpfx}"} )
+  fi
+}
 
-  # Check for `--` within command-line and quit or strip suffix.
-  local i
-  for i in "${!words[@]}"; do
-    if [ "${words[$i]}" == "--" ]; then
-      # Do not complete words following `--` passed to CLI_ARGS.
-      [ $cword -gt $i ] && return
-      # Remove the words following `--` to not put --list in CLI_ARGS.
-      words=( "${words[@]:0:$i}" )
-      break
+_task() {
+  local cur prev words cword
+
+  # Completion directives, mirroring internal/complete/complete.go.
+  local -ri NO_SPACE=2 NO_FILE_COMP=4 FILTER_FILE_EXT=8 FILTER_DIRS=16 KEEP_ORDER=32
+
+  # `=` and `:` out of the word breaks: `--output=`, `docs:serve` stay one token.
+  _init_completion -n =: || return
+
+  local -a args=( "${words[@]:1:cword}" )
+  if (( ${#args[@]} == 0 )); then
+    args=( "" )
+  fi
+
+  local output
+  output=$("$TASK_CMD" __complete "${args[@]}" 2>/dev/null)
+  if [[ -z "$output" ]]; then
+    _task_filedir
+    return
+  fi
+
+  local -a lines=()
+  local line
+  while IFS= read -r line; do
+    lines+=( "$line" )
+  done <<< "$output"
+
+  local last_idx=$(( ${#lines[@]} - 1 ))
+  local directive="${lines[$last_idx]#:}"
+  unset 'lines[$last_idx]'
+
+  if (( directive & FILTER_FILE_EXT )); then
+    local exts=""
+    # ${arr[@]+…} guards an empty array under `set -u` in bash 3.2 (macOS).
+    for line in ${lines[@]+"${lines[@]}"}; do
+      exts+="${exts:+|}$line"
+    done
+    _task_filedir "@($exts)"
+    return
+  fi
+
+  if (( directive & FILTER_DIRS )); then
+    _task_filedir -d
+    return
+  fi
+
+  # Not `compgen -W`: it splits the word list on IFS, mangling values with spaces.
+  local value
+  COMPREPLY=()
+  for line in ${lines[@]+"${lines[@]}"}; do
+    value="${line%%$'\t'*}"
+    if [[ -z "$cur" || "$value" == "$cur"* ]]; then
+      COMPREPLY+=( "$value" )
     fi
   done
 
-  # Handle special arguments of options.
-  case "$prev" in
-    -d|--dir|--remote-cache-dir)
-      _filedir -d
-      return $?
-    ;;
-    --cacert|--cert|--cert-key)
-      _filedir
-      return $?
-    ;;
-    -t|--taskfile)
-      _filedir yaml || return $?
-      _filedir yml
-      return $?
-    ;;
-    -o|--output)
-      COMPREPLY=( $( compgen -W "interleaved group prefixed" -- $cur ) )
-      return 0
-    ;;
-  esac
+  if (( directive & NO_SPACE )); then
+    compopt -o nospace 2>/dev/null
+  fi
 
-  # Handle normal options.
-  case "$cur" in
-    -*)
-      COMPREPLY=( $( compgen -W "$(_parse_help $1)" -- $cur ) )
-      return 0
-    ;;
-  esac
+  # nosort needs bash 4.4; the 3.2 shipped by macOS ignores it and stays sorted.
+  if (( directive & KEEP_ORDER )); then
+    compopt -o nosort 2>/dev/null
+  fi
 
-  # Prepare task name completions.
-  local tasks=( $( "${words[@]}" --silent $_GO_TASK_COMPLETION_LIST_OPTION 2> /dev/null ) )
-  COMPREPLY=( $( compgen -W "${tasks[*]}" -- "$cur" ) )
-
-  # Post-process because task names might contain colons.
   __ltrim_colon_completions "$cur"
+
+  if (( ${#COMPREPLY[@]} == 0 )) && ! (( directive & NO_FILE_COMP )); then
+    _task_filedir
+  fi
 }
 
 complete -F _task "$TASK_CMD"