Problem

SessionManager::list_sessions (crates/tui/src/session_manager.rs:306) reads and JSON-parses every session file in ~/.deepseek/sessions/ to build a Vec<SessionMetadata>. cleanup_old_sessions (line 346) calls list_sessions then sorts and prunes — meaning every cleanup pays the full parse cost for files that are about to be deleted anyway.

For a user with hundreds of historical sessions, this lags the startup path and any session-listing UI.

Fix

Add a metadata-only path:

• Read each session file's first ~4KB only (enough to JSON-parse the header: id, created_at, last_modified, turn_count, model, etc).
• For cleanup_old_sessions, use the metadata-only path — we don't need full session contents to decide what to delete.
• Keep the full-parse path for load_session.

Verify against the existing tests (test_list_sessions at line 586) and add one for the metadata-only path.

Acceptance criteria

• New list_sessions_metadata() returns header-only metadata without full parse.
• cleanup_old_sessions switches to the metadata path.
• Existing list_sessions and tests untouched (load + parse semantics preserved for callers that need the full session).
• Add a benchmark or smoke test demonstrating the speedup with ≥50 fixture sessions.
• Standard verification gates pass.