Keep parent tab highlighted when focus is on a sub-agent

The top tab bar compared against focusedID, so stepping into a
sub-agent dropped the parent tab's highlight even though the user
was still inside that thread. activeAgentID already walks the
parent chain to the top-level root for the sidebar's agent tree
— reuse it for the tab strip too.

CHANGELOG.md

@@ -6,6 +6,13 @@ loosely follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Fixed
+- Top tab bar now keeps the top-level agent's tab highlighted
+  when focus is on one of its sub-agents (or on a Processes pane
+  entry, matching the existing agent-tree behavior). Previously
+  the tab would lose its highlight as soon as you stepped into a
+  child agent, even though you were still within that thread.
+
 ## [0.0.2] - 2026-05-15
 
 ### Added