refactor(desktop): extract use-prompt-actions standalone helpers into utils

[OutThisLife]

Summary

Fifth in the apps/desktop god-file split series. usePromptActions is the textbook "god hook" AGENTS.md's TS guidance warns against. As a first, safe slice, this pulls the hook's module-level standalone helpers (no closure over hook state) into a focused, testable use-prompt-actions-utils.ts sibling.

use-prompt-actions.ts: 1,956 → 1,772 lines.

Moved into use-prompt-actions-utils.ts

• error classifiers: isSessionNotFoundError, isSessionBusyError, isProviderSetupError, inlineErrorMessage
• session-busy retry: withSessionBusyRetry (+ SESSION_BUSY_RETRY_* constants)
• attachment IO: base64FromDataUrl, imageFilenameFromPath, readImageForRemoteAttach, readFileDataUrlForAttach, friendlyRemoteAttachError
• misc: delay, isSessionIdCandidate, blobToDataUrl, renderCommandsCatalog, slashStatusText, appendText, visibleUserOrdinal, visibleUserIndexAtOrdinal, the _submitInFlight guard set, and the GatewayRequest type

Notes

• Pure restructuring — no behavior change. Helpers moved verbatim; the usePromptActions and uploadComposerAttachment exports (and their import paths in thread.tsx / desktop-controller.tsx / the test) are unchanged.
• A deeper decomposition of the hook body (slash dispatch, submit/retry, attachment upload, restore targeting) into colocated sub-hooks is left for a dedicated follow-up — that's a stateful refactor, not a pure move.

Test plan

• npm run typecheck (desktop) — passes
• eslint on changed/added files — clean
• vitest run for use-prompt-actions-utils.test.ts + the existing use-prompt-actions.test.tsx — 51 pass
• Smoke-test: submit, queue drain, slash commands (/help, /model, …), remote attachment upload, message edit/restore

Note: a whole-tree npm run lint reports 4 pre-existing errors in unrelated files on main; none are touched here.

[tonydwb]

Code Review Summary\n\nVerdict: LGTM\n\nClean god-file split — use-prompt-actions.ts reduced from 1,956 to 1,772 lines by extracting standalone helpers into use-prompt-actions-utils.ts. Pure restructuring with no behavior change. Tests pass (51 unit tests). Well-scoped 3-file change with clear module boundaries.\n\n### Looks Good\n- Verbatim move of standalone helpers with no logic changes\n- New test file covers the extracted functions\n- TypeCheck and ESLint clean\n---\nReviewed by Hermes Agent

[tonydwb]

Code Review: LGTM

Clean extraction of prompt-action utility functions into a dedicated use-prompt-actions-utils.ts module. All exported functions have unit tests covering edge cases (session ID candidates, error message parsing, data URL extraction, image filename extraction, etc.). The parent hook imports from the new module without behavioral change.

Verdict: LGTM -- well-tested god-file reduction.