refactor: shorten file mentions in thread titles

[neriousy]

What Changed

Mentioning a file at the start would insert the full file path into the thread name.
This PR makes it so rather than inserting the full file path, only the @filename.tsx gets inserted in there

Why

Having the full file path in there was making it hard to distinguish threads (yeah you can still rename them but it was annoying to look at)

UI Changes

First thread is before, second is after

## 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

Note

Replace truncateTitle with buildThreadTitle to shorten @file mentions in thread titles

• Introduces buildThreadTitle which replaces full file paths in @mention segments with their basenames before truncating to 50 characters with an ellipsis.
• Removes truncateTitle and its tests, replacing all call sites in ChatView with the new utility.
• Behavioral Change: Thread titles that include @file mentions now display only the filename (e.g. @foo.ts) rather than the full path before truncation.

^{Macroscope summarized 1d7307f.}

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[Copilot]

Pull request overview

This PR replaces the generic truncateTitle helper with a thread-specific buildThreadTitle that normalizes composer @path mentions to their basenames before applying truncation, then updates thread creation flows to use it.

Changes:

• Removed truncateTitle and its unit tests.
• Added buildThreadTitle with mention-aware normalization + new unit tests.
• Updated ChatView to use buildThreadTitle for new thread titles (including plan implementation threads).

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
apps/web/src/truncateTitle.ts	Removed the old generic title truncation helper.
apps/web/src/truncateTitle.test.ts	Removed tests for the deleted helper.
apps/web/src/threadTitle.ts	Added mention-aware thread title normalization + truncation.
apps/web/src/threadTitle.test.ts	Added coverage for mention shortening + truncation ordering.
apps/web/src/components/ChatView.tsx	Switched thread-title generation to buildThreadTitle.

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