Fix namespaced skill install file tree output - #14254

Fixes #14190

Description

Namespaced skills install into flat directories, but the post-install file tree still resolves their qualified names as nested paths. Successful installs therefore display (could not read directory).

Resolve each file-tree entry from its flat on-disk name while preserving the qualified name in the success message. The InstallName comment now describes the returned name instead of the obsolete nested install layout.

How did you test this change?

Given a local skill at src/skills/alice/deploy/SKILL.md, I ran:

bin/gh skill install ./src --from-local --all --force --dir target

Before, the install succeeded but displayed:

Installed alice/deploy (from ./src) in target

  alice/deploy/
  (could not read directory)

After, it displayed the installed directory and file:

Installed alice/deploy (from ./src) in target

  deploy/
  └── SKILL.md

The installed file remained at target/deploy/SKILL.md.

Key points

• Only the post-install file tree on stderr changes; the install itself and its file layout are untouched.
• Success messages and the lockfile continue to use the qualified name (alice/deploy). Only the tree label and the directory it reads use the flat name.
• printFileTree derives the flat name from the qualified name it already receives, so the installer's Result shape stays unchanged.

Notes for reviewers

The behavior change is in printFileTree, which both the repository and --from-local install paths call. The regression assertion extends the existing namespaced local-install table case.

#13266 introduced the flat install layout that this output now follows.

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:

• @username will read and reply directly. Name the account.
• An agent will draft replies and @Hiro5409 will read them before they are posted.
• Nobody has explicitly committed to replying.