M7 #4: split isAgentWorking into isGenerating + isPostProcessing

Pass-1 showed the "Agent is working…" spinner persisting long after
the reply had landed in the message list — Hermes delays the ACP
`promptComplete` event while it does auxiliary post-work (title
generation, usage accounting). Spinner stuck ~minute+ on a 2-second
response.

Fix without touching the ACP state machine: derive two computed
properties from existing signals in RichChatViewModel:

- `isGenerating`: agent is working AND we don't yet have a finalized
  assistant reply on the message list. Drives the prominent spinner.
- `isPostProcessing`: agent is working AND the user CAN see the
  reply. Drives a subtle "Finishing up…" pill instead of the big
  spinner. When `promptComplete` finally arrives, `isAgentWorking`
  flips false and both derived props go quiet.

`isAgentWorking` remains the canonical ACP-level flag (kept public
for any consumer that really wants the raw value), just no longer
the signal for visible "spinner now" UX.

Applied to:
- ScarfGo ChatView.swift — primary spinner + post-processing pill.
- Mac RichChatView.swift — SessionInfoBar + RichChatMessageList now
  take `isGenerating` instead of `isAgentWorking`. Same UX win for
  the macOS app (pass-1 finding was cross-platform, just surfaced
  first on iOS).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scarf/Packages/ScarfCore/Sources/ScarfCore/ViewModels/RichChatViewModel.swift

@@ -44,11 +44,41 @@ public final class RichChatViewModel {
     public var messages: [HermesMessage] = []
     public var currentSession: HermesSession?
     public var messageGroups: [MessageGroup] = []
+    /// True from the moment the user sends a prompt until the ACP
+    /// `promptComplete` event arrives. Covers the whole round-trip
+    /// including auxiliary post-processing (title generation, usage
+    /// accounting, etc.). UIs should prefer the `isGenerating` /
+    /// `isPostProcessing` pair below — they distinguish "agent is
+    /// thinking about your message" from "agent is closing out" and
+    /// avoid the misleading "spinner after the reply has landed" UX
+    /// we saw in pass-1 (M7 #4).
     public var isAgentWorking = false
     public var pendingPermission: PendingPermission?
     /// Mutated to trigger a scroll-to-bottom in the message list.
     public var scrollTrigger = UUID()
 
+    /// True while the assistant hasn't yet emitted a complete reply
+    /// for the latest user prompt. Renders the prominent "Agent is
+    /// thinking…" indicator in the chat. Flips false as soon as we've
+    /// finalized an assistant message with content — even if the ACP
+    /// `promptComplete` event hasn't arrived yet (Hermes auxiliary
+    /// work like title generation delays that event).
+    public var isGenerating: Bool {
+        isAgentWorking && !isPostProcessing
+    }
+
+    /// True while ACP hasn't closed out the prompt but the assistant
+    /// has already finalized a reply the user can see. Renders a
+    /// subtle "Finishing up…" pill instead of the prominent spinner.
+    /// Avoids the pass-1 M7 #4 UX where users stared at "Agent is
+    /// working…" forever because `promptComplete` was held up by
+    /// auxiliary server-side work.
+    public var isPostProcessing: Bool {
+        guard isAgentWorking else { return false }
+        guard let last = messages.last else { return false }
+        return last.isAssistant && !last.content.isEmpty
+    }
+
     // Cumulative ACP token tracking (ACP returns tokens per prompt but DB has none)
     public private(set) var acpInputTokens = 0
     public private(set) var acpOutputTokens = 0