[FEATURE]: Allow API clients and agents to open existing sessions as Desktop tabs #53529

Feature hasn't been suggested before.

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Describe the enhancement you want to request

OpenCode's API can create, move, and prompt sessions, but it cannot ask the Desktop client to expose an existing session as a visible tab. Desktop tab state is client-local, so automated workflows can prepare sessions but still require the user to find and Ctrl+click each one on Home.

Please add a supported mechanism—such as a Desktop API action, deep link, or UI intent—to open one or more existing sessions as Desktop tabs.

Suggested semantics:

{
  "sessions": [
    { "sessionID": "ses_...", "background": true }
  ]
}

• background: true: add the tab while keeping the current tab active.
• background: false: add and activate the tab.
• Accept multiple sessions in one request.
• Reuse/focus an already-open tab instead of creating a duplicate.
• Opening a tab must not prompt, resume, or otherwise execute the session.
• The Desktop client should validate that the session belongs to its configured server and is accessible.

Use case: an agent inventories multiple worktrees, creates or organizes one session per work item, and prepares them as visible Desktop tabs for the user to review and continue interactively. Creating server-side session records alone does not complete this workflow.

Related:

• Add a deep link to open a specific Desktop session #44167 proposes deep links for opening an existing Desktop session and may provide the narrow technical foundation.
• [FEATURE]: Generic UI Intent Channel for cross-client plugin-driven UX #6330 discusses a generic cross-client UI-intent channel, which could potentially carry an open-session intent.
• feat(desktop): reopen closed tabs and background tab open #35010 added manual background opening with Ctrl+click, which is the desired behavior but is not programmatically accessible.