refactor(tui): store-driven turn state + slash registry + module split

Hoist turn state from a 286-line hook into $turnState atom + turnController
singleton. createGatewayEventHandler becomes a typed dispatch over the
controller; its ctx shrinks from 30 fields to 5. Event-handler refs and 16
threaded actions are gone.

Fold three createSlash*Handler factories into a data-driven SlashCommand[]
registry under slash/commands/{core,session,ops}.ts. Aliases are data;
findSlashCommand does name+alias lookup. Shared guarded/guardedErr combinator
in slash/guarded.ts.

Split constants.ts + app/helpers.ts into config/ (timing/limits/env),
content/ (faces/placeholders/hotkeys/verbs/charms/fortunes), domain/ (roles/
details/messages/paths/slash/viewport/usage), protocol/ (interpolation/paste).

Type every RPC response in gatewayTypes.ts (26 new interfaces); drop all
`(r: any)` across slash + main app.

Shrink useMainApp from 1216 -> 646 lines by extracting useSessionLifecycle,
useSubmission, useConfigSync. Add <Fg> themed primitive and strip ~50
`as any` color casts.

Tests: 50 passing. Build + type-check clean.

ui-tui/src/domain/slash.ts

@@ -0,0 +1,25 @@
+export interface ParsedSlashCommand {
+  arg: string
+  cmd: string
+  name: string
+}
+
+export const looksLikeSlashCommand = (text: string) => {
+  if (!text.startsWith('/')) {
+    return false
+  }
+
+  const first = text.split(/\s+/, 1)[0] || ''
+
+  return !first.slice(1).includes('/')
+}
+
+export const parseSlashCommand = (cmd: string): ParsedSlashCommand => {
+  const [rawName = '', ...rest] = cmd.slice(1).split(/\s+/)
+
+  return {
+    arg: rest.join(' '),
+    cmd,
+    name: rawName.toLowerCase()
+  }
+}