fix: keep conversation titles valid UTF-8 in promptToTitle truncation

[eminemead]

• ListConversations failed to marshal when a stored conversation title contained invalid UTF-8.
• promptToTitle truncated with a byte slice (title[:47]), which split multi-byte runes.
• Truncation is now rune-safe, and titles are sanitized with strings.ToValidUTF8 at the marshal boundary (invalid bytes become U+FFFD) so previously stored rows still list.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

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Developed in collaboration with Claude Code