docs: refresh planner priorities for 4208

[asim]

Summary:

• Remove completed first-agent on-ramp item closed by Verify installed first-agent on-ramp #4207.
• Promote the open AtlasCloud guarded-delegate conformance recurrence as the top Now-phase risk, followed by provider cancellation/retry and durable checkpoints.

Testing:

• git diff --check
• go build ./...
• go test ./... (fails in this environment because loopback gRPC tests are blocked by envoy: "Loopback targets forbidden")
• golangci-lint run ./...

Closes #4208