Address maintainer re-review nits:
- Changeset body no longer claims install-exports resolution (Failure 2 is
now handled upstream by #1511 / ADR-1508 Phase 2). This PR delivers the
.gsd-source marker writer, which fixes list/status (Failure 1).
- Add negative tests locking the `runtime === 'claude' && isGlobal` write
guard: a non-claude global install and a claude *local* install both
provision no marker. Verified must-fail-first against a relaxed guard.
Address review on #1487:
- Scope the marker write to runtime === 'claude' && isGlobal, matching
issue #1477. The Claude global skills layout is the only install path
that ships the skills layout without a commands/gsd source tree; every
other runtime/scope deploys commands/gsd, so walk-up already resolves.
- Add the trailing (#1487) reference to the changeset body per PRED.k329.body.
Claude-Session: https://claude.ai/code/session_01XNT3SWgzjmEycNuweURDme
