fix(cli): prevent snapshot progress session hangs - #13945

What Problem This Solves

Snapshot initialization could remain visible after a mid-session snapshot operation, and an interrupted progress part could be replayed into the next provider request. With snapshots disabled, later operations could also wait behind a stale snapshot lock.

Closes #13638

Why This Change Was Made

• Preserve the project Effect context for delayed snapshot progress updates and cleanup.
• Skip snapshot lock acquisition when snapshots are disabled.
• Exclude persisted transient snapshot progress parts from provider history conversion.

User Impact

Snapshot progress clears in the correct session and project. Continuing or forking a session no longer replays the synthetic initialization text into model history, and disabled snapshots do not wait on snapshot locks.

Evidence

• 39 focused snapshot, history, and fork regression tests pass.
• CLI typecheck passes.
• Extension build passes.
• Isolated Agent Manager testing passed for new, continued, and forked sessions with snapshots disabled.
• Annotation and formatting checks pass.