refactor(worker): collapse session goroutines into single runSession loop

Replace the five-goroutine session model (RunDialer, RunKeepalive,
RunReader, RunHeartbeatForwarder, RunStopMonitor, Session) with a single
DefaultWorker.runSession method containing two select loops: one
pre-connection and one connected. Ephemeral dial goroutines replace
RunDialer; the keepalive timer and heartbeat reset are inlined. No
exported building-block symbols remain.

Consolidate worker_dialer_test.go, worker_session_test.go, and
worker_start_test.go into worker_test.go. Add seven new behavioral
tests covering dial failure, keepalive-driven dial replacement,
pre-connection stop, message delivery with timestamp, sustained
activity and pong resetting the keepalive timer, keepalive-triggered
reconnect, and nil connection pointer after disconnect.
Update EXTEND.md and README.md to remove references to the deleted

building blocks and document the single worker replacement pattern

AGENTS.md

@@ -0,0 +1,4 @@
+# go-honeybee
+
+## Build
+- Run `go fmt` on every edited file before staging.