feat(threads): auto-generate first-turn thread titles

[maria-rcks]

Closes #990

What Changed

• use the existing text generation model setting for first-turn thread title generation
• auto-generate a thread title from the first user prompt on the server
• keep title generation off the critical path so the turn starts immediately
• rename the settings copy from "Git writing model" to "Text generation model"
• reuse the same text generation model for automatic worktree branch naming

Why

Thread titles were still just truncated prompt text.

This makes new threads easier to scan while keeping the model selection simpler and reusing the same text generation path we already have for commit, PR, and branch text.

UI Changes

Settings now says Text generation model

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

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Note

Medium Risk
Moderate risk because it changes first-turn orchestration behavior and contracts by adding a new textGenerationModel field and triggering additional async side effects (title generation + branch rename) that could affect thread metadata updates.

Overview
Auto-generates a thread title from the first user turn. On the server, ProviderCommandReactor now forks a new first-turn side effect that calls TextGeneration.generateThreadTitle and dispatches thread.meta.update with the generated title.

Extends the text-generation pipeline and plumbing. TextGeneration adds generateThreadTitle, implemented in CodexTextGeneration with attachment-aware prompting plus sanitization (single-line, quote/whitespace normalization, 50-char truncation, and fallback to New thread). The client and contracts now pass an optional textGenerationModel through thread.turn.start → thread.turn-start-requested, and that model is reused for automatic worktree branch naming.

UI copy update. The settings label/description for the model selector is renamed from "Git writing model" to "Text generation model" to reflect expanded usage.

^{Written by Cursor Bugbot for commit af6a180. This will update automatically on new commits. Configure here.}

Note

Auto-generate thread titles on the first user turn using the configured text generation model

• On the first user message in a thread, ProviderCommandReactor now forks a maybeGenerateThreadTitleForFirstTurn call alongside the existing worktree branch rename, dispatching a thread.meta.update with the generated title on success.
• A new generateThreadTitle method is added to TextGenerationShape (implemented in CodexTextGeneration), which builds a prompt from the message text and any attachments, calls Codex, and returns a sanitized title.
• Title sanitization trims whitespace, strips surrounding quotes/backticks, collapses internal whitespace, falls back to 'New thread' if empty, and truncates to 50 characters with an ellipsis.
• The textGenerationModel field is threaded through the orchestration contract (ThreadTurnStartCommand, ThreadTurnStartRequestedPayload) and the ChatView UI dispatch so both title and branch generation use the user-configured model.
• The settings label for this model is renamed from 'Git writing model' to 'Text generation model' to reflect its broader use.

^{Macroscope summarized af6a180.}

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