FN-7628: allow editing sent chat messages and rewinding agent responses

Adds the ability to edit a previously sent message in an agent chat, which rewinds the session/task room and regenerates the response from the edited message.

- Add chat-store support for locating/replacing a message and truncating subsequent history for a rewind
- Add a chat-manager rewind-session flow and a new register-chat-routes endpoint to rewind a room to an edited message
- Add legacy API route wiring and useChat hook support for issuing an edit request
- Add ChatView/StandardChatSurface/TaskPlannerChatTab UI affordances (edit control, styling) to trigger message edits
- Add a changeset documenting the new chat message-edit capability
- Add unit/integration tests covering chat-store rewind logic, chat-manager rewind-session behavior, chat routes, useChat, and ChatView edit UI

Files changed:
 .changeset/fn-7628-chat-message-edit.md            |   7 +
 docs/dashboard-guide.md                            |   4 +
 packages/core/src/__tests__/chat-store.test.ts     | 171 ++++++++++++++
 packages/core/src/chat-store.ts                    |  95 ++++++++
 packages/dashboard/app/api/legacy.ts               |  22 ++
 packages/dashboard/app/components/ChatView.css     |  78 ++++++
 packages/dashboard/app/components/ChatView.tsx     |  14 ++
 .../app/components/StandardChatSurface.tsx         |  87 ++++++-
 .../app/components/TaskPlannerChatTab.tsx          |   8 +
 .../__tests__/ChatView.autosize.test.tsx           |   1 +
 .../__tests__/ChatView.default-model-icon.test.tsx |   1 +
 .../components/__tests__/ChatView.draft.test.tsx   |   1 +
 .../__tests__/ChatView.hash-mention.test.tsx       |   1 +
 .../__tests__/ChatView.message-edit.test.tsx       | 262 +++++++++++++++++++++
 .../__tests__/ChatView.mobile-render.test.tsx      |   1 +
 .../components/__tests__/ChatView.rooms.test.tsx   |   1 +
 .../__tests__/ChatView.scroll-to-top.test.tsx      |   1 +
 .../components/__tests__/ChatView.test-harness.tsx |   1 +
 .../dashboard/app/hooks/__tests__/useChat.test.ts  |  98 ++++++++
 packages/dashboard/app/hooks/useChat.ts            |  60 +++++
 .../__tests__/chat-manager-rewind-session.test.ts  | 185 +++++++++++++++
 .../dashboard/src/__tests__/chat-manager.test.ts   |  12 +
 .../dashboard/src/__tests__/chat-routes.test.ts    | 124 ++++++++++
 packages/dashboard/src/chat.ts                     | 147 +++++++++++-
 .../dashboard/src/routes/register-chat-routes.ts   |  52 ++++
 25 files changed, 1429 insertions(+), 5 deletions(-)

Fusion-Task-Id: FN-7628

Fusion-Task-Lineage: 36d98989-1b75-428c-baf1-b2c7e8e78013

Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai>

packages/dashboard/app/api/legacy.ts

@@ -10188,6 +10188,28 @@ export function deleteChatMessage(
   );
 }
 
+/**
+ * FNXC:ChatMessageEdit 2026-07-07-09:00:
+ * Edit an earlier user message in a direct (model-loop) chat session. Truncates the persisted
+ * transcript from (and including) the target message onward AND rewinds the pi session context
+ * server-side, so the returned `retained` list is the surviving pre-edit history. Does NOT
+ * trigger regeneration — the caller resends the edited content via the existing streaming send.
+ */
+export function editChatMessage(
+  sessionId: string,
+  messageId: string,
+  content: string,
+  projectId?: string,
+): Promise<{ retained: ChatMessage[] }> {
+  return api<{ retained: ChatMessage[] }>(
+    withProjectId(`/chat/sessions/${encodeURIComponent(sessionId)}/messages/${encodeURIComponent(messageId)}`, projectId),
+    {
+      method: "PATCH",
+      body: JSON.stringify({ content }),
+    },
+  );
+}
+
 export function fetchChatRooms(
   options: { status?: string; agentId?: string } = {},
   projectId?: string,