[deploy-plugin DPB-21] Thin platform clients: fly, koyeb, sevalla, coolify, dokploy + live smokes

[rickylabs]

Part of #892 (RFC #891; corpus .llm/runs/plan-deploy-plugin--seed/ r5).

Subpath REST clients mapping the generic platform port — managed: Fly Machines API, Koyeb, Sevalla; self-hosted: Coolify, Dokploy (user-supplied base URL + token env names). Live smokes on ≥1 managed and ≥1 self-hosted platform. Anti-scope: a client graduates to its own package only per the OF-2 rule (SDK, auth, lifecycle/error semantics, release cadence, or live-gate ownership divergence).

Acceptance

• gate: one shared conformance run per client
• gate: live smoke green on one managed + one self-hosted platform (probe lane)

Metadata

Dependencies: DPB-20
Delivery shape: subpath REST clients