Summarize chat trigger (#1890)

<!-- CURSOR_SUMMARY -->
> [!NOTE]
> Adds a context-limit banner with one-click “summarize into new chat,”
refactors token counting with react-query, and persists per-message max
token usage.
> 
> - **Chat UX**
> - **Context limit banner** (`ContextLimitBanner.tsx`,
`MessagesList.tsx`): shows when within 40k tokens of `contextWindow`,
with tooltip and action to summarize into a new chat.
> - **Summarize flow**: extracted to `useSummarizeInNewChat` and used in
chat input and banner; new summarize system prompt
(`summarize_chat_system_prompt.ts`).
> - **Token usage & counting**
> - **Persist max tokens used per assistant message**: DB migration
(`messages.max_tokens_used`), schema updates, and saving usage during
streaming (`chat_stream_handlers.ts`).
> - **Token counting refactor** (`useCountTokens.ts`): react-query with
debounce; returns `estimatedTotalTokens` and `actualMaxTokens`;
invalidated on model change and stream end; `TokenBar` updated.
> - **Surfacing usage**: tooltip on latest assistant message shows total
tokens (`ChatMessage.tsx`).
> - **Model/config tweaks**
> - Set `auto` model `contextWindow` to `200_000`
(`language_model_constants.ts`).
>   - Improve chat auto-scroll dependency (`ChatPanel.tsx`).
>   - Fix app path validation regex (`app_handlers.ts`).
> - **Testing & dev server**
> - E2E tests for banner and summarize
(`e2e-tests/context_limit_banner.spec.ts` + fixtures/snapshot).
> - Fake LLM server streams usage to simulate high token scenarios
(`testing/fake-llm-server/*`).
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
2ae16a14d50699cc772407426419192c2fdf2ec3. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->













<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Adds a “Summarize into new chat” trigger and a context limit banner to
help keep conversations focused and avoid hitting model limits. Also
tracks and surfaces actual token usage per assistant message, with a
token counting refactor for reliability.

- **New Features**
- Summarize into new chat from the input or banner; improved system
prompt with clear output format.
- Context limit banner shows when within 40k tokens of the model’s
context window and offers a one-click summarize action.
  - Tooltip on the latest assistant message shows total tokens used.

- **Refactors**
- Token counting now uses react-query and returns estimatedTotalTokens
and actualMaxTokens; counts are invalidated on model change and when
streaming settles.
- Persist per-message max_tokens_used in the messages table; backend
aggregates model usage during streaming and saves it.
- Adjusted default “Auto” model contextWindow to 200k for more realistic
limits.
- Improved chat scrolling while streaming; fixed app path validation
regex.

<sup>Written for commit 2ae16a14d50699cc772407426419192c2fdf2ec3.
Summary will update automatically on new commits.</sup>

<!-- End of auto-generated description by cubic. -->

src/prompts/summarize_chat_system_prompt.ts

@@ -1,8 +1,42 @@
 export const SUMMARIZE_CHAT_SYSTEM_PROMPT = `
-You are a helpful assistant that understands long conversations and can summarize them in a few bullet points.
+You are a helpful assistant that summarizes AI coding chat sessions with a focus on technical changes and file modifications.
 
-I want you to write down the gist of the conversation in a few bullet points, focusing on the major changes, particularly
-at the end of the conversation.
+Your task is to analyze the conversation and provide:
 
-Use <dyad-chat-summary> for setting the chat summary (put this at the end). The chat summary should be less than a sentence, but more than a few words. YOU SHOULD ALWAYS INCLUDE EXACTLY ONE CHAT TITLE
+1. **Chat Summary**: A concise summary (less than a sentence, more than a few words) that captures the primary objective or outcome of the session.
+
+2. **Major Changes**: Identify and highlight:
+   - Major code modifications, refactors, or new features implemented
+   - Critical bug fixes or debugging sessions
+   - Architecture or design pattern changes
+   - Important decisions made during the conversation
+
+3. **Relevant Files**: List the most important files discussed or modified, with brief context:
+   - Files that received significant changes
+   - New files created
+   - Files central to the discussion or problem-solving
+   - Format: \`path/to/file.ext - brief description of changes\`
+
+4. **Focus on Recency**: Prioritize changes and discussions from the latter part of the conversation, as these typically represent the final state or most recent decisions.
+
+**Output Format:**
+
+## Major Changes
+- Bullet point of significant change 1
+- Bullet point of significant change 2
+
+## Important Context
+- Any critical decisions, trade-offs, or next steps discussed
+
+## Relevant Files
+- \`file1.ts\` - Description of changes
+- \`file2.py\` - Description of changes
+
+<dyad-chat-summary>
+[Your concise summary here - less than a sentence, more than a few words]
+</dyad-chat-summary>
+
+**Reminder:**
+
+YOU MUST ALWAYS INCLUDE EXACTLY ONE <dyad-chat-summary> TAG AT THE END.
 `;