Add completion/tests/ harnesses that exercise the engine protocol and every
shell wrapper without a TTY: the engine via `task __complete`, bash/zsh by
stubbing the completion-system helpers, fish via `complete -C`, and PowerShell
via the completion API. A `test:completion` task and a matching CI job (with a
strict mode that fails when an expected shell is missing) run them all.
Writing the suite surfaced and fixed real wrapper bugs:
- fish: `math` has no bitwise `&`, so every directive check errored; test bits
with integer division + modulo instead. Also filter FilterFileExt results
ourselves, as __fish_complete_suffix only prioritizes the extension.
- powershell: `Get-ChildItem -Include` is ignored without -Recurse, so file
extension filtering returned nothing; filter with Where-Object instead.
- bash: guard empty-array expansion so completion with zero candidates does not
trip `set -u` on bash 3.2.
Also name the completion directive bits (NO_SPACE, FILTER_FILE_EXT, …) in every
wrapper instead of using raw numbers.
Engine:
- Emit DirectiveKeepOrder for task variables so the `requires` declaration
order is preserved instead of being sorted by the shell.
- Return full `--flag=value` candidates for the inline `--output=` form so all
shells match against the whole current token.
- Add `--no-aliases` / `--no-descriptions` completion flags (via complete.Options)
parsed from the __complete invocation; the zsh wrapper maps its show-aliases
and verbose zstyles onto them.
- Skip Taskfile setup when completing flags (NeedsTaskfile) and load the task
list lazily; drop unused parameters; document exported identifiers.
Shell wrappers:
- zsh: reindent to tabs (.editorconfig), bridge zstyles to engine flags.
- fish: reindent to 2 spaces, handle every file-completion directive in the
wrapper (--no-files disables the native fallback), drop the duplicated
yml/yaml extension list now that it lives only in the engine.
- bash: prefix-filter by hand to preserve values containing spaces, exclude `=`
from word breaks so `--output=` reaches the engine as one token.
- powershell: filter candidates by the current word, fall back to file
completion for DirectiveDefault.
NoSpace is not representable in fish/PowerShell completion APIs; documented in
the wrappers.
