storage: opening a live sessions.db with the sqlite3 CLI makes the daemon report disk I/O errors / corrupt the file #4513

Observed

During the #4497 verification (isolated daemon, release build), opening a live sessions.db with the sqlite3 CLI while the daemon was running was followed each time by the daemon failing with disk I/O error, and once the file later reported file is not a database.

Why it matters

Users and agents routinely inspect stores with sqlite3 while the daemon runs. A WAL-mode SQLite database should tolerate a concurrent external reader. If the daemon's locking/VFS/WAL settings (exclusive locking mode, custom VFS, checkpoint/truncate behavior, PRAGMA choices) make an external reader corrupt the store, that is data loss.

Expected

Either an external read-only sqlite3 session is safe, or the daemon refuses it with a clear lock error; it must never corrupt the store. Reproduce with a controlled test and identify which setting allows it. Related: #4152, #4481 (corruption warnings).