Upgrade httpok to remove the Redis dependency

[piraz]

Overview

Upgrade github.com/candango/httpok from v0.1.0 to v0.2.0 so NvimIM no
longer pulls the Redis session backend into its dependency graph.

httpok remains a valid dependency through gopeasant; it does not need to
be removed from the project.

Requirements

• Upgrade httpok to v0.2.0 or a later Redis-free release.
• Keep the existing gopeasant protocol integration working.
• Preserve the current httpok test helper unless it becomes incompatible.
• Confirm that Redis is no longer present in the module and build dependency
  graph.

Acceptance criteria

• go list -deps ./... contains no Redis packages.
• go mod tidy leaves no Redis module in go.mod or go.sum.
• go test ./... and go vet ./... pass.
• GitHub release fetching and protocol tests remain functional.