feat(completion): add 'ant completion <shell>' command

[EvanPaules]

Add a user-facing ant completion <shell> command that prints a shell completion script to stdout. Supports bash, zsh, fish, and pwsh.

The completion-script renderer and the embedded shell-script templates were already shipping under the hidden @completion command, used by the release pipeline (.goreleaser.yml). This change makes that capability discoverable to end users via a single canonical surface.

Changes:

• internal/autocomplete: add BuildCompletionCommand(appName) *cli.Command, the single source of truth for the completion command tree. Both production (pkg/cmd) and tests construct the command through this factory. Consolidates what were three near-identical "specify a shell" / "unsupported shell" error templates into one usageError helper.

• pkg/cmd: register the command in cmd_completion.go (an 11-line shim matching the hand-written-command pattern of cmd_auth.go / cmd_profile.go). Remove the hidden @completion registration in cmd.go - the public command supersedes it.

• .goreleaser.yml: invoke ant completion <shell> instead of @completion <shell> for the release-bundled completion scripts.

• .gitattributes (new): force LF line endings on internal/autocomplete/shellscripts/* so Windows checkouts cannot introduce CRLF into the embedded scripts, which would break sourcing on Linux/macOS for binaries built from such checkouts.

• internal/autocomplete/shellscripts/zsh_autocomplete.zsh: update an inline comment that referenced the removed @completion form.

• internal/autocomplete/command_test.go (new): integration tests that drive the production factory directly - each shell renders correctly, bare completion returns a usage error, unknown shells return an error naming the supported set, and the renderer rejects unsupported shells with a usable message. Replaces the previous pkg/cmd/cmd_completion_test.go (its locally-defined helpers no longer exist after the consolidation).

Notes for reviewers:

• The previously-hidden @completion command was renamed away rather than aliased, so the public surface is the only entry point. The hidden __complete runtime-completion command is unchanged.

• pwsh is exposed publicly alongside bash/zsh/fish even though goreleaser only packages the latter three - this keeps one source of truth (SupportedShells in the autocomplete package). Restricting the public command set is a one-line filter if maintainers prefer.

• urfave/cli's Suggest: true is set on the completion parent for consistency with sibling commands; on this command it governs only flag suggestions, not unknown-subcommand suggestions, because PrefixMatchCommands is not enabled. The hand-rolled usageError message is what users see for unknown shells.

[EvanPaules]

Is there anyone who can review this?