Stop probing a sentinel document in grid mode

[hamzamerzic]

Summary:

• use the new null-path idle document contract while no note is open
• preserve the stable hook options introduced by the immediate durable-save work
• remove the synthetic sentinel filename from the storage namespace
• release Notes 1.2.30

Verification:

• rebased over Notes 1.2.29 immediate durable saves
• production bundle rebuilt
• all 230 tests passed against the landed platform runtime with zero skips
• null path performed no reads, subscriptions, or writes
• existing durable write, offline queue, dead-letter, merge, and revision tests remain green