Clarify skill publish error for nested skill directories - #14259

Closes #14134

Description

When gh skill publish is given a skill directory containing SKILL.md directly, shared discovery represents that skill's path as .. Publish then compares the skill name with . and reports the misleading error name "reskill" does not match directory name ".".

This change recognizes only that already-failing case when the selected skill directory is inside a Git repository and replaces the name-mismatch error with an explanation that gh skill publish must target the repository root. It does not add general repository-root enforcement or change successful publishing behavior.

How did you test this change?

Given this checkout contains skills/gh-skill, when I ran go run ./cmd/gh skill publish skills/gh-skill --dry-run, then the command still exited unsuccessfully but explained that the selected path is a skill directory within a repository and that publishing must target the repository root.

Key points

• Checks only a single directly supplied skill represented by discovery as ..
• Preserves the existing failure and changes only its explanation.
• Leaves all other directory targeting and publishing behavior unchanged.

Notes for reviewers

Start with the targeted check in publishRun, followed by TestPublishRun_ExplainsSkillDirectoryWithinRepository. #14134 contains the original misleading diagnostic.

Authorship and follow-up

Who wrote this:

• A human wrote it.
• An agent wrote it under close human direction.
• An agent wrote it independently, and no human has guided the implementation beyond the initial prompt.

Who answers review comments:

• @tommaso-moro will read and reply directly. Name the account.
• An agent will draft replies and @username will read them before they are posted.
• Nobody has explicitly committed to replying.