[Bug] Onboarding guide sharing broken in 2.1.215: `check` crashes on e.length, `update`/`create` report success but write nothing

Bug Description

The onboarding-guide share tool (ShareOnboardingGuide) is broken end-to-end in 2.1.215 (current latest). All three modes fail, in two distinct ways:

1. check mode crashes before uploading, with what looks like an unparsed API response in the guide-listing step:

[unavailable] Upload didn't go through (undefined is not an object (evaluating 'e.length')). Fall back to the manual share copy.
Reproduced 3x across ~1 hour, identical error each time.

1. update (with a valid short_code) and create report success but write nothing. Both return the success template with unpopulated fields:

[updated] Share link updated: undefined (short_code: undefined)
[created] Share link created: undefined (short_code: undefined)
After the "successful" update, the hosted guide page still serves the previous content - verified in a logged-in browser session with a hard reload and a cache-busting query param, ~10 minutes after the call. If create made a new guide server-side, its URL is unrecoverable because the response is unparsed and check (the only list operation) is the mode that crashes.

Steps to Reproduce

1. Have an existing org onboarding guide (created weeks earlier, working link).
2. Place an updated ONBOARDING.md in the working directory.
3. Call the onboarding share tool with mode: check -> e.length crash.
4. Call with mode: update, short_code: <existing guide's code> -> "updated ... undefined"; hosted page unchanged.
5. Call with mode: create -> "created ... undefined"; no way to discover the new URL.

Expected

check uploads to the most-recent guide and returns its link; update/create return the real link and short_code; the hosted page serves the new content.

Impact

Org onboarding links cannot be refreshed at all - ours currently serves stale setup instructions to new team members, with no workaround (no UI surface for guide management found on claude.ai, and no newer CLI version to update to). Likely side effect: orphaned guides created by the failed update/create attempts.

Environment Info

• Platform: darwin (macOS 25.5.0)
• Terminal: tmux (Ghostty)
• Version: 2.1.215
• Feedback ID: dc0a501b-0e49-4c9f-a839-13ee263a6d43 (submitted via /bug with session context)

The affected guide's short_code and timestamps available on request.