fix: Render at-mentioned chips as @labels in task titles

[charlesvien]

Problem

At-mentioned directories (and other chip types) rendered as raw XML like <folder path="products/agentic_tests" /> in task titles instead of human-readable text.

Closes #2172

Changes

1. Add xmlToPlainText helper composing xmlToContent + contentToPlainText
2. Use it in the task-creation saga so every chip type renders as @label
3. Drop unused createFileTagRegex import from the saga
4. Update attachment-only saga test (now shows @tmp/code.ts instead of "Reading attachment…")
5. Add folder-mention regression test plus xmlToPlainText unit tests

How did you test this?

Manually

Publish to changelog?

[charlesvien]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• fix: Render at-mentioned chips as @labels in task titles #2268 👈 (View in Graphite)
• fix: Buffer feature-flag subscribers registered before init #2267 : 1 other dependent PR (#2270 )
• feat: Add nightly cleanup of stale draft releases #2239
• main

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