[BUG] Cowork save_skill returns bare "Turn ended" with no success/error and installs nothing

What's wrong

In Cowork, save_skill sometimes ends with only "Turn ended". There is no success response, no error and no approval prompt, and nothing is installed. Reloading the skill shows the old text. An immediate retry of the identical call succeeds with a full upload response, and later saves in the same session work normally.

Steps to reproduce

1. In a Cowork session, call save_skill to overwrite an existing skill with new content.
2. Observe the tool result: only "Turn ended".
3. Reload the skill: the old content is still installed.
4. Retry the identical call: it succeeds.

Intermittent. Seen once in three saves in one session.

Expected

Every call returns either a success result or an explicit error. A silent no-op is indistinguishable from a successful save unless the user reloads and diffs.

Also noticed (minor)

• A leading or trailing newline in content adds a blank line to the installed file.
• An unquoted name: value in the frontmatter comes back quoted.

Environment

• Windows 11 Pro 10.0.26200
• Claude desktop app 2.16120.0.0 (Cowork)
• 2026-09-30

Related but different: #68071 (Claude Code can't save to the Skills store at all).