[FEATURE]: support delivery: "queue" for session.shell #52274

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Describe the enhancement you want to request

In v2, session.prompt, session.command and session.compact accept delivery: "steer" | "queue", but session.shell does not. A user shell command always runs immediately, in parallel with the active execution.

await client.session.prompt({ sessionID, text: 'fix the test', delivery: 'queue' }) // waits for the current turn
await client.session.shell({ sessionID, command: 'git commit -am wip' })             // runs now, no delivery option

Why it matters

I build a Discord client on top of the v2 server (kimaki). Users can type !cmd to run a shell command in the session, and !cmd. queue to run it after the current turn. Common case:

• agent is still editing files
• user queues !git commit -am wip or !pnpm test
• running it now commits or tests half-done work

Today the client has to keep its own queue and call session.shell on session.execution.succeeded. That queue can't be ordered against the native inbox: with prompt A (queue), !test (queue), prompt B (queue), opencode starts B right after A, before the client runs !test.

Proposal

Add an optional delivery to session.shell, same semantics as prompts:

POST /api/session/:sessionID/shell
{ id?: string, command: string, delivery?: "steer" | "queue" }

• steer (default): current behavior, run now
• queue: admit a durable inbox item, run it at the next queue boundary in order with other queued items, then continue with the next item
• the shell result still becomes the resume: false synthetic message, so it lands in context without waking the agent
• session.inbox.enqueued / delivered / cancelled events cover it, so clients can show and cancel queued shells like queued prompts

Session.shell already routes the result through synthetic(...) (packages/core/src/session/session.ts), so this is mostly about admitting it through the inbox instead of running it immediately.