fix(session): ensure agent exists before processing title in session summary

[0x404]

the title agent may be disabled, and if it's disabled will lead to error:

TypeError: Cannot read properties of undefined

Fixes #8663

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This PR doesn't have a linked issue. All PRs must reference an existing issue.

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1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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

Based on my search, I found one potentially related PR:

#8635 - Fix TUI crashes from undefined values (#8588, #8579, #8580)
#8635

This PR addresses similar issues with undefined values causing crashes in the TUI. It may be related to the same underlying problem of checking for agent existence before processing, though it appears to focus on a different component (TUI vs session summary).

All other search results either return the current PR itself (#8662) or are unrelated feature PRs that don't address the specific issue of handling disabled/undefined agents.