skills: Gmail helper calls the API via curl so it (#398)

works behind an https CONNECT egress proxy The Gmail skill helper's HTTP layer used Python urllib, which cannot tunnel through an https:// CONNECT egress proxy, so in sandboxes routed through one every helper call failed and agents fell back to hand-rolled plain-text sends — losing the multipart/alternative HTML mirror and producing mail that recipients see re-wrapped at a narrow width. The helper's call() now shells out to curl, which tunnels CONNECT proxies natively; MIME construction is untouched, and non-2xx or non-JSON responses exit cleanly with the status and a body excerpt. Stub-curl tests assert the request shape (URL, auth header, JSON body over stdin), response parsing, and error behavior.

**Deployment notes**

Release note: existing sandboxes keep the old gmail.py until re-provisioned/re-seeded
skills-seed script change only (gmail.py urllib→curl). New sandboxes get the fix at seed time;
boxes provisioned before the upgrade keep the stale copy until their skill tree is re-seeded — the
fork ran a manual sprite sweep for this; upstream needs a release note or an automatic re-seed
path for existing sandboxes.
Adds a runtime expectation that curl exists in the sandbox image (universally true in practice).

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