fix: hide subagent threads from v2 lists

[Yusuf007R]

Problem

When an agent calls delegate_task, the server creates a child thread shell and marks it with lineage.relationshipToParent === "subagent". The V2 Web and Mobile thread lists were not honoring that relationship, so the delegated child appeared as a separate top-level thread beside its parent.

That duplicate entry also leaked into list bookkeeping: subagents could affect active/settled/snoozed counts and, on Web, could reorder projects by activity even though the child row was hidden.

Fix

• Filter subagent shells before Web Sidebar V2 and Mobile Thread List V2 partition, count, and snooze calculations.
• Use the same visible-thread set for Web project activity sorting.
• Keep root threads and user-created forks visible; only relationshipToParent === "subagent" is excluded.
• Keep the delegated task accessible from its card inside the parent conversation.

Result

Before, the parent and delegated child were both shown as top-level threads. After, only the parent appears in the sidebar.

Before

After

Verification

• 97 focused Web and Mobile list tests passed.
• Final Web sidebar logic suite: 85 tests passed.
• Web and Mobile typechecks passed.
• Targeted formatting, lint, and diff checks passed.
• Verified the delegated-child flow in an isolated Web environment.
• Two independent review agents found no remaining issues.

Scope

Client-side behavior only; no protocol or persistence changes. This PR targets the branch from PR #2829.

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[cursor]

Cursor Bugbot has reviewed your changes using high effort and found 1 potential issue.

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[macroscopeapp]

Approvability

Verdict: Approved

This PR adds UI-level filtering to hide subagent threads from V2 list views. The change is simple (checking relationshipToParent === 'subagent'), well-tested, and builds on existing helper functions with no schema or security implications.

^{You can customize Macroscope's approvability policy. Learn more.}

Dismissing prior approval to re-evaluate 5da5f72