fix: read responses on outgoing Primary connections

[qj0r9j0vc2]

Summary

• Fixed critical bug in TcpPrimaryNetwork::connect_to_peer() where the reader half of outgoing TCP connections was immediately dropped
• Previously: let (_, writer) = tokio::io::split(stream); - reader assigned to _ and dropped
• Now: spawns a reader task to handle incoming messages (e.g., attestations) on outgoing connections

Root Cause

When Primary A connects to Primary B and sends a Car:

1. A initiates connection to B (outgoing from A's perspective)
2. A sends Car via the writer half
3. B receives Car on its incoming connection handler
4. B generates attestation and sends it back
5. BUG: A's reader was dropped, so A never receives the attestation
6. A times out waiting for attestation: "Car attestation failed after max retries"

Test plan

• cargo build -p cipherd passes
• cargo test -p cipherd passes (85 tests)
• cargo fmt -- --check passes
• cargo clippy -p cipherd -- -D warnings passes
• Deploy to testnet and verify attestations are received