feat(completion): unify shell completions behind task __complete

[vmaerten]

Summary

Adds a shell completion engine: a single task __complete command written in Go, that drives Bash, Fish, Zsh and PowerShell.
The shell scripts become thin wrappers that forward the current words to task __complete and render the suggestions, so every shell gets the same, richer behaviour and there is no more per-shell drift.

The wire protocol mirrors cobra v2 (value\tdescription lines followed by a :<directive> bitfield) behavior.

Why

It's painful to maintain 4 (or more) different completion scripts, and it's also difficult to test them. With this approach, all new flags will be included in every shell by default.

What it bring more than old completion

It's not only a drop in replacement of the old completions.
Now we can have requires vars being autocompleted, and even with enum 🚀

Testing

Following the cobra approach, the protocol is tested in Go and the generated scripts are only smoke-tested for how they route each directive:

• internal/complete/complete_test.go: white-box tests of the engine.
• completion/protocol_test.go: black-box test of the real binary's __complete output (candidates + directive).
• completion/tests/: thin per-shell smokes (bash/zsh/fish/powershell) that assert directive routing (no-files, dir-only, extension filter, inline --flag=, …), runnable via task test:completion.
• A completion CI job runs the smokes on Ubuntu and macOS (with fish and PowerShell installed, strict mode).

Disclaimer: I used AI to write the completion scripts & the testing protocol (in bash).

[vmaerten]

@arturict, please don't comment on draft PRs. Also, your comments appear to be AI-generated. We're still open to meaningful, human-written comments. Finally, please avoid giving instructions to maintainers as if you were one of them.

[arturict]

Understood, and sorry about that. I should not have commented on a draft or phrased the comment as an instruction. I'll step back.