feat(cli): add positional slug shortcuts for publish workflows

[copieman]

Summary

The ship CLI already supports targeting apps by slug via --app-slug and SHIP_APP_SLUG. A useful next step is to make the common publish flows even shorter by accepting a positional slug as shorthand.

Proposed UX

Support full publish with a positional slug:

ship <slug>
ship publish <slug>

Support partial/status/validation flows with a positional slug:

ship publish metadata <slug>
ship publish screenshots <slug>
ship publish app <slug>
ship publish status <slug>
ship publish validate <slug>

Preserve existing explicit targeting:

ship publish --app-id <uuid>
ship publish --app-slug <slug>
ship publish <slug> --platform ios

Why

• The public CLI already supports slug-based targeting, but the most common path is still fairly verbose.
• Positional slug syntax makes publish commands easier to remember and faster to type.
• It gives the CLI a cleaner “happy path” while keeping --app-id and --app-slug available for explicit workflows.

Scope

• Add positional slug support to the root command for full publish.
• Add positional slug support to publish and the publish-related subcommands.
• Keep --app-id, --app-slug, and platform filtering working as they do today.
• Update help text and README examples to document the shorthand flows.

Acceptance Criteria

• ship <slug> triggers a full publish for the matching app.
• ship publish <slug> triggers a full publish for the matching app.
• metadata, screenshots, app, status, and validate accept <slug> as an optional positional argument.
• Existing --app-id and --app-slug flows continue to work.
• Platform selection still works with shorthand commands.
• The CLI returns clear validation errors when the user supplies conflicting identifiers.
• README and command examples reflect the new shorthand syntax.

Notes

• There is already a local prototype branch for this UX: feature/slug-publish-shortcuts.
• The issue should describe the desired public behavior, not commit us to the exact prototype implementation.