feat(hooks): emit session:start and session:end lifecycle events

[bayrakdarerdem]

Summary

The gateway hook system has had session:start listed in its documentation since day one — but it was never actually emitted. Any plugin author who wrote a hook listening for session:start would get silence. This PR fixes that disconnect and adds a companion session:end event that was also missing.

Changes
gateway/run.py

Emits session:start on the first message of a new session (detected by checking if transcript history is empty)
Emits session:end just before session:reset so hooks can clean up or persist data before the session is destroyed

gateway/hooks.py

Updated docstring to accurately list both new events
Clarified the distinction between session:end (session is closing) and session:reset (new session entry has been created)

Why This Matters
The hook system is designed for extensibility — plugins subscribe to lifecycle events and react accordingly. A session boundary is one of the most fundamental events in any agent system. Without session:start and session:end, plugins have no way to:

Initialize per-session state
Persist session analytics
Send welcome or goodbye messages
Clean up resources when a conversation ends

This is a small but foundational fix for the plugin ecosystem.