Promote the prefork frames to a shared worker protocol module

[pratyush618]

Move the line-delimited JSON frames from crates/taskito-python/src/prefork/protocol.rs into crates/taskito-core/src/worker/protocol.rs so one wire format serves both the pipe and socket transports, and migrate the prefork pool onto it.

Add three frames:

Frame	Direction	Payload
hello	executor to scheduler	{executor_id, sdk, version, tasks[], slots}
hello_ack	scheduler to executor	{scheduler_id, protocol_version}
heartbeat	executor to scheduler	{free_slots}

Payload bytes stay CBOR per crates/taskito-core/BINDING_CONTRACT.md. Protocol version mismatch is rejected at hello_ack, never silently downgraded.

Done when cargo test --workspace is green and prefork behaviour is unchanged. First phase — everything else builds on it. Plan: tasks/executor-attach.md.