Add dist-tag command

[thelovekesh]

Add dist-tag command with add sub-command to set dist tags.

[github-actions]

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[Copilot]

Pull request overview

Adds a new wpm dist-tag command group (with an add subcommand) to let users set distribution tags for already-published package versions, and wires it into the CLI, registry client, shell completions, and reference docs.

Changes:

• Introduces wpm dist-tag add PACKAGE@VERSION [TAG] and registers it in the root command set.
• Extends the registry client with AddDistTag to PUT dist-tag updates to the registry API.
• Updates docs and reuses a unified dist-tag completion function for both publish --tag and dist-tag add.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
pkg/pm/registry/client.go	Adds AddDistTag API call and request payload type.
docs/reference/cli/wpm.md	Adds dist-tag to the CLI reference index.
docs/reference/cli/dist-tag.md	New reference page for the dist-tag command group.
docs/reference/cli/dist-tag_add.md	New reference page for dist-tag add usage and examples.
cli/command/publish/publish.go	Switches publish tag completion to the renamed DistTags() helper.
cli/command/disttag/cmd.go	New dist-tag command group with alias dist-tags.
cli/command/disttag/add.go	Implements dist-tag add, argument parsing, auth validation, and output.
cli/command/completion/functions.go	Renames PublishTags → DistTags and updates its doc comment.
cli/command/commands/commands.go	Registers the new dist-tag command with the CLI.

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