fix(tui): route /save through session.save JSON-RPC

The cherry-picked approach serialized the UI-shaped transcript on the Node
side, producing a third JSON format alongside cli.py save_conversation and
tui_gateway session.save. Simpler to call the existing session.save method,
which already writes the canonical agent history (raw OpenAI messages +
model) to an absolute-path file.

- /save still short-circuits before the slash worker
- Empty transcript -> 'no conversation yet'
- No active session -> 'no active session - nothing to save'
- Otherwise: rpc('session.save', {session_id}) and echo back the file path
- Tests updated to assert RPC contract; new test covers the no-sid case

ui-tui/src/app/slash/commands/core.ts

@@ -1,5 +1,3 @@
-import { writeFileSync } from 'node:fs'
-
 import { NO_CONFIRM_DESTRUCTIVE } from '../../../config/env.js'
 import { dailyFortune, randomFortune } from '../../../content/fortunes.js'
 import { HOTKEYS } from '../../../content/hotkeys.js'
@@ -7,6 +5,7 @@ import { isSectionName, nextDetailsMode, parseDetailsMode, SECTION_NAMES } from
 import type {
   ConfigGetValueResponse,
   ConfigSetResponse,
+  SessionSaveResponse,
   SessionSteerResponse,
   SessionUndoResponse
 } from '../../../gatewayTypes.js'
@@ -48,24 +47,6 @@ const DETAILS_USAGE =
 
 const DETAILS_SECTION_USAGE = 'usage: /details <section> [hidden|collapsed|expanded|reset]'
 
-const pad2 = (n: number): string => String(n).padStart(2, '0')
-
-const saveTimestamp = (d = new Date()): string =>
-  `${d.getFullYear()}${pad2(d.getMonth() + 1)}${pad2(d.getDate())}_${pad2(d.getHours())}${pad2(
-    d.getMinutes()
-  )}${pad2(d.getSeconds())}`
-
-const serializableTranscript = (items: Msg[]) =>
-  items
-    .filter(m => m.role === 'user' || m.role === 'assistant' || m.role === 'tool')
-    .filter(m => m.text.trim() || m.thinking?.trim() || m.tools?.length)
-    .map(m => ({
-      role: m.role,
-      text: m.text,
-      ...(m.thinking ? { thinking: m.thinking } : {}),
-      ...(m.tools?.length ? { tools: m.tools } : {})
-    }))
-
 export const coreCommands: SlashCommand[] = [
   {
     help: 'list commands + hotkeys',
@@ -375,33 +356,32 @@ export const coreCommands: SlashCommand[] = [
     help: 'save the current transcript to JSON',
     name: 'save',
     run: (_arg, ctx) => {
-      const messages = serializableTranscript(ctx.local.getHistoryItems())
+      const hasConversation = ctx.local
+        .getHistoryItems()
+        .some(m => m.role === 'user' || m.role === 'assistant' || m.role === 'tool')
 
-      if (!messages.length) {
+      if (!hasConversation) {
         return ctx.transcript.sys('no conversation yet')
       }
 
-      const filename = `hermes_conversation_${saveTimestamp()}.json`
-
-      try {
-        writeFileSync(
-          filename,
-          `${JSON.stringify(
-            {
-              model: ctx.ui.info?.model ?? null,
-              saved_at: new Date().toISOString(),
-              session_id: ctx.sid,
-              messages
-            },
-            null,
-            2
-          )}\n`,
-          'utf8'
-        )
-        ctx.transcript.sys(`conversation saved to: ${filename}`)
-      } catch (error) {
-        ctx.transcript.sys(`failed to save: ${String(error)}`)
+      if (!ctx.sid) {
+        return ctx.transcript.sys('no active session — nothing to save')
       }
+
+      ctx.gateway
+        .rpc<SessionSaveResponse>('session.save', { session_id: ctx.sid })
+        .then(
+          ctx.guarded<SessionSaveResponse>(r => {
+            const file = r?.file
+
+            if (file) {
+              ctx.transcript.sys(`conversation saved to: ${file}`)
+            } else {
+              ctx.transcript.sys('failed to save')
+            }
+          })
+        )
+        .catch(ctx.guardedErr)
     }
   },