feat(agents): Select the active session in the Agents list (#140)

Switching between agent session tabs in the editor now moves the Agents
tool window's selection to that task, expanding its group (e.g. Archived)
when it is collapsed. The list also opens with the active session selected.
Tabs of other kinds leave the selection as it is.

The editor service gains activeInput() and onActiveInputChange() so
features can follow the active tab of any kind.

src/workbench/contributions.ts

@@ -409,6 +409,10 @@ export interface EditorService {
   openPaths?(): string[];
   /** Every open tab, files or not, most recently active first (Switcher). */
   openTabs?(): OpenEditorTab[];
+  /** Input of the active tab (any kind), if any. */
+  activeInput?(): EditorInput | null;
+  /** Calls `fn` whenever the active tab changes; returns the unsubscribe. */
+  onActiveInputChange?(fn: (input: EditorInput | null) => void): () => void;
   /** Closes every tab showing `path`. */
   closePath?(path: string): void;
   /** Closes the tab of the input with this `key` (e.g. a merge editor once applied). */