agent-shell 0.78 updates

The latest version of agent-shell, 0.78, brings several improvements and new features. A standout feature is the persistent prompt, which offers a writeable prompt at all times in the buffer, allowing users to submit prompts and immediately receive responses while the AI agent handles the request. This persistent prompt is enabled by default and can be disabled if desired.

Another significant update is the ability for agent-shell to steer turns in AI agents that support the _session/steering ACP extension, enabling users to course correct in-flight prompts without waiting for processing to complete. Users can now steer by submitting with M-RET, or queue by pressing RET. The viewport's compose buffer remains functional, with prompts being handled through the same logic.

Additionally, agent-shell can now handle file dragging and dropping from external file managers, with images receiving a preview and other files being attached as @path links. Token cost headers have been added to the session display, showing cumulative cost when available. The editor now provides more idomatic slash command completion across the shell prompt, viewport compose buffer, and minibuffer.

The M-x agent-shell-copy-last-output command allows users to quickly retrieve the most recent output from anywhere in the buffer. Various bug fixes and improvements have also been incorporated, including fixes to file mentions carrying whitespace in paths, improved table rendering, and enhanced markdown support.