v0.3.9.beta

Added in-flight conversation steering to the web UI — while the agent is streaming, a "Steer" button appears once a tool call begins, allowing users to inject guidance via the steerConversation API without interrupting the run. Also includes a "Stop" button to abort an active stream.
Fixed steering message injection order for Google and OpenAI Responses providers so pending steer messages are appended after the current user message (previously they could be processed too early).
Extracted MaxMessageLength constant in pkg/steer and referenced it from the CLI, and fixed the empty-state layout in ChatTranscript to fill the full viewport height.