docs: refresh planner priorities for 4201

[asim]

Summary:

• Remove completed exact-once AtlasCloud plan-delegate item closed by Enforce idempotent plan-delegate notifications #4200.
• Promote the installed first-agent on-ramp as the top developer-adoption priority.
• Keep provider cancellation/retry next and add durable agent checkpoint resume as the following Next-phase issue.

Testing:

• git diff --check && go build ./... && go test ./... (go test fails in this environment because loopback targets are forbidden in grpc client/transport tests)
• golangci-lint run ./...

Closes #4201