[FEATURE]: Search session contents from session pickers

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Describe the enhancement you want to request

Session search in the desktop Home screen / Cmd+B picker and the TUI /sessions picker currently matches session titles only. Generated titles are often insufficient when trying to recover a particular discussion, error message, command, or decision from an older session.

Please allow session search to match user and assistant text stored throughout each session. Desktop results could show one compact context snippet with the matching phrase highlighted, while TUI results can remain compact.

Search should respect the selected project scope and continue to support global search from Home. Results should also remain stable while typing.

This would make past discussions discoverable without requiring users to remember generated titles or open and export sessions individually.

Related prior requests: #11819 and #14708 were closed for inactivity, and their associated PR #14718 was closed unmerged. #15602 concerns searching within the currently open session and only mentions cross-session search as future work. This request is specifically scoped to searching stored message text from the session picker, with compact matching snippets.