feat(opencode): heuristic fallback for title generation

[Retromadee]

Adds a heuristic fallback when the small model used for title generation fails. The fallback uses the first user message trimmed to 60 characters, or a generic placeholder. This improves robustness of session title creation.

[github-actions]

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

I found 2 potentially related PRs that address similar title generation issues:

1. fix(core): use catalog small model for session titles #36563 - fix(core): use catalog small model for session titles

   • Related to title generation using the small model

2. fix(opencode): stop silent session title generation failures #35440 - fix(opencode): stop silent session title generation failures

   • Directly addresses handling title generation failures

These PRs may be exploring related or previous attempts to solve title generation issues. You may want to review them to ensure this new PR (38968) doesn't duplicate their work or should build upon them instead.

[github-actions]

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