perf(agent): scope read_workspace to the requested section

[kvnloo]

Problem

The model's read_workspace tool currently calls the full workspace snapshot for every request, then throws most of it away.

For section: "mail", that still loads:

• calendar events;
• files;
• browser sessions;
• actions;
• activity;
• profile/token metadata.

The same over-read happens for calendar and files requests.

A downstream PGlite benchmark with mail 2000 / events 500 / files 200 / browsers 300 / actions 800 / activity 1000 measured the mail path at roughly 291.9ms / 2.6MB → 254.8ms / 2.0MB, with calendar/files sections avoiding >90% of unrelated transfer.

Change

Add WorkspaceService.sectionSnapshot() for the three tool-visible source sections and make read_workspace use it.

The existing full snapshot() remains the full workspace API and still performs one connection check. Shared mail/event loading preserves:

• live Google fetches;
• mail caching;
• event persistence;
• existing sorting and query filtering.

A files-only read does not touch Google, browsers, actions, activity, profile, or runtime metadata.

Regression

Persistence coverage compares mail/calendar/files section results against the corresponding fields from the existing full snapshot.

No connector permissions, tool schema, or returned section payload changes.