refactor(desktop): decompose use-prompt-actions (slash + submit sub-hooks)

[OutThisLife]

Summary

God-hook split #3 of 3 — and the deepest. usePromptActions' standalone helpers already shipped (#55459) + foldered (#55493), so what remained was the stateful body. Its single biggest unit is executeSlashCommand: a ~530-line /command dispatcher (with a recursive inner runSlash).

This lifts it into a colocated useSlashCommand sub-hook (use-prompt-actions/slash.ts). The ~13 values it closed over become a typed SlashCommandDeps object the parent passes in; the dispatcher body (and its inner runSlash recursion + nested handlers) moves verbatim. The slash-only SlashActionCtx type moves with it.

index.ts: 1,772 → 1,212 lines (slash.ts is 614).

Notes

• Pure restructuring — the dispatcher body is unchanged; only the closure surface is now an explicit deps object. tsc verifies every reference is threaded (a missing dep = a compile error).
• Behavior-preserving, verified: the full use-prompt-actions test suite (51 tests) still passes, including the real usePromptActions hook test that drives slash commands.
• usePromptActions keeps its other callbacks (submit/retry, attachment upload, restore/edit, steer, cancel) and now does const executeSlashCommand = useSlashCommand({ … }).

Test plan

• npm run typecheck — 0 errors
• eslint on the folder — clean
• vitest run use-prompt-actions — 51 pass
• Smoke: /help, /model, /skin, a skill command, an alias (e.g. recursive dispatch), /undo prefill, an exec command that renders inline output

[OutThisLife]

Extended: also lifted the ~280-line submitPromptText pipeline into a useSubmitPrompt sub-hook (submit.ts); SubmitTextOptions moved to utils.ts. index.ts is now 1,772 → 937. typecheck/eslint clean; full 51-test suite green.