fix(tui): guard prompt submit against concurrent invocation

[kitlangton]

Summary

Two submit() calls in packages/opencode/src/cli/cmd/tui/component/prompt/index.tsx can overlap (most commonly a double-pressed Enter). Both pass the if (!store.prompt.input) return false guard, both await sdk.client.session.create(...), and both only read inputText = store.prompt.input after that await. The first call finishes, sends its prompt, then clears the store; the second call, now past its await, reads "" and sends an empty prompt to a second freshly-created session.

The user observes:

• An orphaned session (visible only by browsing) containing their actual text and a real assistant reply.
• A phantom session — the one they navigate to — with an empty user message, an assistant reply (the model responds to nothing), and a hallucinated title (the title generator inferred from its own system prompt because the user content was empty).

Reproduced in a real session in the OpenCode DB: two session rows created 3ms apart, one with text: "Hello there." and one with text: "", sharing the same directory.

Fix

Wrap submit() in an in-flight submitting flag. The original body is preserved verbatim as submitInner(); only the guard is new.

let submitting = false
async function submit() {
  if (submitting) return false
  submitting = true
  try {
    return await submitInner()
  } finally {
    submitting = false
  }
}

Tests

packages/opencode/test/cli/tui/prompt-submit-race.test.ts is a structural mirror of submit(). The first commit adds it pointed at the buggy shape (no guard) and fails. The second commit adds the production fix and updates the mirror to match, after which both tests pass.

Caveat: this is a structural regression test, not an integration test against the real Prompt component. Mounting Prompt for a true E2E test would require building harnesses for ~14 contexts (sync, sdk, project, editor, dialog, toast, command-palette, keymap, tui-config, route, args, local, prompt-history, prompt-stash) plus a fake SDK client. Happy to do that as a follow-up if reviewers want it.

Test plan

• bun run test test/cli/tui/prompt-submit-race.test.ts from packages/opencode (2 pass)
• bun run typecheck from packages/opencode (clean)
• Manual: in the TUI, type a message, double-press Enter quickly, verify exactly one session is created and contains the typed text