Agents Window: ignored blocked sessions reappear after reload

## Bug
In the Agents Window, ignoring a blocked session only lasts for the current window lifetime. Reloading the window discards the acknowledgement, so the same pending input request or failing CI occurrence appears again in the title-bar requires-input indicator/dropdown even though the user already ignored it.

## Steps to reproduce
1. Use an Insiders or development build with the Agents Window (the blocked-session indicator is not enabled in Stable).
2. Have a non-visible session blocked on a pending approval/question, or associated with an open, non-draft pull request whose CI is failing. For the CI case, ensure Agent Merge is not handling CI fixes for that session.
3. Open the title-bar requires-input dropdown and ignore that session. Confirm it disappears from the dropdown.
4. Keep that session non-visible and leave the pending request/failing commit unchanged.
5. Run **Developer: Reload Window** and wait for the session provider and CI/approval data to load.

**Actual:** The same ignored occurrence is surfaced again.

**Expected:** The ignored occurrence remains suppressed after reload. A genuinely new input request or failing commit can still surface. Archiving the session removes its saved acknowledgement, and persisted history is limited to the 50 most recent acknowledged sessions.

## Reliability considerations
Restoration must preserve acknowledgements while cached sessions and approval models hydrate, and while remote providers disconnect/reconnect. Shared profile storage must not let an older window overwrite another window's acknowledgements or carry state into a different profile.

Start by tracing the Agents Window acknowledgement state through window reload, session-provider hydration, and approval or CI data restoration. Verify that ignored occurrences remain suppressed across reloads and reconnects, while new occurrences still appear, archiving clears saved acknowledgements, history is capped at 50 sessions, and profile or window state cannot overwrite unrelated acknowledgements.

Written by the indexing model from the issue text.