Add JSON output for skills list (#2658)

* Add JSON output for skills list

* test(cli): resolve tempDir via realpath for macOS compatibility

On macOS, os.tmpdir() returns /var/folders/... but process.cwd()
reports the symlink-resolved /private/var/folders/... after chdir.
The JSON output uses the resolved cwd, so the test assertion mismatched
the unresolved tempDir on macOS.

* chore: add changeset for skills list --json

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Co-authored-by: Fahreddin Özcan <ozcanfahrettinn@gmail.com>