fix(skills): scope What's New pill to Installed tab + reword updated→changed

Issue #78 — The "What's New" pill at the top of the Skills page
announced "18 new, 3 updated since you last looked" while the Updates
sub-tab simultaneously said "No Updates / All skills are up to date."
Two surfaces measuring two different things both used the word
"update": the pill counts local file deltas since the user last
clicked "Mark as seen", while the Updates body runs `hermes skills
check` to find skills with newer upstream versions available. From
the user's seat the screen contradicted itself.

Two changes:

1. Render the pill only on the Installed sub-tab (Mac + ScarfGo).
   Local file deltas are contextually meaningful only on the tab
   that surfaces installed skills; showing them above Browse Hub or
   Updates was misleading.

2. Reword the pill: "X updated since you last looked" → "X changed
   since you last looked". Keeps `SkillSnapshotDiff.updatedCount` as
   the field name (it's still about file changes, not version bumps);
   only the user-visible string changes. Removes the vocabulary
   collision with the Updates tab's separate upstream-update check.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scarf/Scarf iOS/Skills/SkillsView.swift

@@ -48,7 +48,12 @@ struct SkillsView: View {
             // picker when the per-server snapshot diff has changes.
             // First-load with no prior snapshot silently primes (no
             // pill, the snapshot just records what's there).
-            if let diff = snapshotDiff,
+            //
+            // Issue #78: scope the pill to the Installed tab. It
+            // describes local file deltas; rendering it on Updates
+            // contradicts the upstream-version-check pane below.
+            if currentTab == .installed,
+               let diff = snapshotDiff,
                diff.hasChanges,
                !diff.previousSnapshotEmpty {
                 whatsNewPill(diff: diff)