Lack of shared-scoped memory for multi-user and automated contexts (groups, channels, cron, subagents).

## Problem

OpenSquilla currently keeps private memory access fail-closed in shared contexts such as group, channel, cron, and subagent sessions.

This prevents private memory leakage, but it leaves a product gap: shared contexts do not yet have their own durable, scoped memory recall.

## Current Behavior

- Direct, DM, private, and webchat sessions can use private memory recall.
- Group and channel sessions cannot read private memory by default.
- Cron and subagent sessions cannot read private memory by default.
- Explicit memory tool allow rules do not revive private memory access in shared contexts.

## Desired Behavior

Add shared-scoped memory so shared contexts can recall memory that belongs to the current group, channel, team, or workspace without reading private memory.

## Acceptance Criteria

- The memory index records visibility and scope metadata, such as `visibility`, `scope_type`, and `scope_id`.
- `memory_search` filters results by the current session scope.
- `memory_get` enforces the same scope checks and cannot bypass scope by path.
- Shared memory has a defined write path or source root.
- Private/direct memory behavior remains unchanged.
- Group/channel tests prove private memory cannot leak.
- Shared/channel tests prove scoped shared memory can be searched and read.

## Notes

The current fail-closed behavior is intentional. Shared memory access should be re-enabled only after scoped shared memory exists.