fix(cli): honor shell argument in completion command

[Cyame]

Issue for this PR

Closes #41232

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

opencode completion fish currently prints a bash or zsh script depending on $SHELL. yargs (pinned at 18.0.0) picks the completion template from the environment and ignores the positional argument, so sourcing the output in fish fails with "Unsupported use of '='" at the first oc prompt.

An upstream yargs bump does not fix this: no released yargs has fish support yet, and the unreleased one still keys off $SHELL only. Details are in #41232.

This replaces yargs' .completion() with a completion [shell] command that:

• honors the positional argument for bash, zsh and fish
• falls back to $SHELL when the argument is omitted
• falls back to bash for unknown values, keeping the previous behavior

The fish template is the one from yargs/yargs#2568 (MIT). --get-yargs-completions is yargs core and is not touched.

How did you verify your code works?

• With SHELL=/bin/zsh, opencode completion fish prints a fish script. Before this change it printed a zsh script.
• completion bash and completion zsh output is unchanged.
• completion tcsh falls back to bash like the old code did.
• opencode --get-yargs-completions c still lists subcommands.
• New tests in test/cli/cmd/completion.test.ts: 8 passing.
• tsgo --noEmit passes.

Screenshots / recordings

No UI changes.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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