refactor(agent): address review comments for window recording support

[Copilot]

Addresses review feedback from PR #1583 on the window recording feature implementation.

Dependency Management

• Updated now-proto-pdu to use v0.4.2 from crates.io for stable, reproducible builds

Task Lifecycle

• Track window monitor task with JoinHandle and await completion on shutdown to prevent race conditions
• Made stop_window_recording async to properly await task termination

Code Quality

• Removed redundant type annotation on run_dvc_io result
• Extracted duplicate timestamp logic into get_current_timestamp() helper
• Fixed module docs to clarify polling is used for title change detection

Bug Fix

• When title tracking is disabled, preserve previous title in snapshot to avoid missing actual window changes:

// Before: updating full snapshot could mask process/exe_path changes
last_snapshot = Some(snapshot);

// After: preserve previous title when tracking disabled
let prev_title = last_snapshot.as_ref().map_or_else(String::new, |s| s.title.clone());
last_snapshot = Some(WindowSnapshot {
    process_id: snapshot.process_id,
    exe_path: snapshot.exe_path.clone(),
    title: prev_title,
});

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