Subagents cannot access chat image attachments available to the parent agent

**Problem:**
Images attached to a VS Code agent-chat message are visible to the parent agent, but background subagents cannot read those same attachments from the filesystem paths supplied by the runtime.

The paths are under:

```text
Code - Insiders/agentSessionData//attachments/
```

The subagent’s file tools treat this directory as content-excluded. Consequently, tasks such as delegating a visual comparison fail even though the parent agent can view the images directly.

**Expected behavior:**
A subagent delegated by the parent should receive safe access to user-provided attachments, ideally through copied session-scoped files or inherited attachment context.

**Actual behavior:**
The subagent can see the attachment paths in its prompt but cannot open them. It must stop due to content-exclusion policy.

**Impact:**
Users cannot reliably delegate image analysis, screenshot comparisons, or other attachment-based work to subagents.

**Suggested improvement:**
When launching a subagent, make user-approved attachments available through a readable, session-scoped artifact path or pass them as native multimodal context.