feat: allow agent to set VS Code conversation title via tool

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

In the VS Code extension, conversation titles are auto-generated from the first user message. There is no way for the AI agent to programmatically set or update the conversation title.

When working on a project, the auto-generated title (e.g., "Review and analyze proje...") doesn't reflect what the conversation actually became about. Users end up with dozens of tabs with unhelpful names.

The agent often has better context about what the conversation is about than the first message suggests — it should be able to propose or set a meaningful title.

Proposed Solution

Add a tool (e.g., SetConversationTitle) or a metadata mechanism that allows the agent to set the conversation tab title in VS Code. This could work like:

1. Agent-initiated: The agent calls a tool to set/update the title based on conversation context
2. Automatic with override: The extension asks the model for a title after the first exchange, and exposes a tool to update it later

Related: #37902 (CLI terminal tab titles — similar need, different surface)

Alternative Solutions

• Let the agent emit a special markdown comment or metadata tag that the extension picks up as a title
• Add a /title slash command that the user can invoke to ask the agent to rename the conversation

Priority

Medium - Would be very helpful

Feature Category

IDE integrations (VS Code)

Use Case Example

User starts a conversation with "review the project" (in Hebrew). The agent does a full project review, then sets the title to "WA Campaigns — Project Review 2026-03-24" instead of the truncated first message.

Additional Context

This is specifically about the VS Code extension conversation panel title, not the CLI terminal tab title (#37902).