V2: align session shell events with Shell service #35018

Summary

Align V2 session shell events/messages with the new Shell service and decide deterministic behavior for shell/prompt concurrency.

Source discussion: https://discord.com/channels/1391832426048651334/1522263729318658181

Decisions / direction

Use existing Shell service types directly. Do not invent parallel session shell types.

session.shell.started { shell: Shell.Info }
session.shell.ended { shell: Shell.Info, output: Shell.Output }

Projected session message should likely reuse the same shapes:

SessionMessage.Shell { shell: Shell.Info, output?: Shell.Output }

Shell.Info already carries ID, status, command, cwd, selected shell executable, output file, pid/exit, metadata, and time. Shell.Output already carries bounded output page, cursor, size, and truncated flag.

Open concurrency question

What should happen when user shell commands are still running and another prompt arrives?

Options discussed:

• Wait for running shell commands before promoting the next prompt, preserving linear chat semantics.
• Promote the prompt without live shell output unless the user explicitly references a shell resource.
• Avoid default running-shell placeholders, because James flagged that this makes model context timing-dependent and replay-hostile.

Provisional direction: keep behavior basic/deterministic now; revisit richer shell references later.