fix(tui): render commands as attachments

[kitlangton]

What

Render slash commands as first-class command attachments instead of exposing their expanded model-facing templates as user prose.

Before / After

Before: submitting /mycmd hello evaluated the command template and persisted only the expanded text. The model received the correct prompt, but the transcript also displayed THIS IS THE EXPANDED TEMPLATE. Argument: hello.

After: the same submission persists { name: "mycmd", arguments: "hello" } as command provenance alongside the expanded model text. Like a user message containing only a skill attachment, the message has no visible prose and renders only the command attachment badge.

How

• Schema adds optional command provenance to durable prompts and projected user messages.
• Core commands retain the evaluated template in text for the model and persist the original command name and arguments separately.
• TUI optimistic rendering, durable replay, queued previews, history, exports, and the App restoration path format the compact invocation from command provenance.
• The normal user-message surface renders the command as an attachment badge, matching skill-only submission while preserving files, skills, pending controls, and message actions.
• Generated client types expose the optional command invocation.

Scope

This does not change command template evaluation or the model-visible prompt. Existing messages without command provenance continue rendering their stored text.

Testing

• bun turbo typecheck --concurrency=3 (pre-push hook, 33 packages)
• bun run test session-prompt.test.ts session-projector.test.ts in packages/core (55 passed)
• bun run test test/mini/session.shared.test.ts test/mini/stream-v2.transport.test.ts test/mini/runtime.queue.test.ts in packages/tui (78 passed)
• App unit and browser suites covering submission and message presentation (553 unit and 60 browser tests passed)
• opencode-drive run ./.scripts/command-display.drive.ts against this worktree, asserting /mycmd hello is visible and the expanded template is absent
• bun run test:bench before and after: all 42 performance unit checks passed both times; Playwright preview startup failed identically both times on the existing bun: ESM loader error

Demo

Deterministic OpenCode Drive reproduction with a simulated model.

Before

After

Flow

sequenceDiagram
    participant UI as TUI / App
    participant Session as Session.command
    participant Store as Inbox / Message projection
    participant Model as Model runner

    UI->>Session: /mycmd hello
    Session->>Session: evaluate command template
    Session->>Store: text = expanded template<br/>command = { name, arguments }
    Store-->>UI: render command attachment
    Store-->>Model: send text

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