[Feature] Full-text search across session messages #706

Problem

The global command palette (Cmd+K) finds projects and sessions by metadata only: session name, label, worktree name, project name. But you often remember what was said in a session, not what it was called.

Today the only way to search message content is Magic → Load Context (src/components/magic/hooks/useLoadContextData.ts), which lowercases and includes()-scans every message of every session client-side. That is acceptable inside a modal you open deliberately; it is not viable as a global, keystroke-responsive search.

Proposal

Real full-text search over session message content, surfaced in the command palette — likely behind a mode or prefix so it does not dilute the fast metadata results — showing a matching snippet with the query highlighted, and jumping to that message inside the session.

Implementation notes

• list_all_sessions (jean-core/src/chat/commands.rs) already returns every message of every session, so the bottleneck is the client-side payload, not data availability.
• A real implementation should search Rust-side via a new search_session_messages command, registered in both src-tauri/src/lib.rs generate_handler![] and src-tauri/src/http_server/dispatch.rs so it works over web access too.
• Re-scanning session JSONL on every keystroke will not scale; this probably wants an index (SQLite FTS5) kept in sync as runs complete.
• Needs debouncing plus snippet extraction with surrounding context.

Acceptance criteria

• Search message content across all projects and worktrees
• Results show project · worktree · session plus a highlighted snippet
• Selecting a result opens the session scrolled to that message
• Stays responsive with thousands of sessions
• Works over web access as well as in the native app

Related

Follow-up to adding session entries to the command palette (recent sessions + metadata search).