perf(header): cap session tabs at 20; tooltip data loads on hover only

Auto-add means the strip only grows, so past 20 tabs the oldest one
leaves the working set (the newly opened tab is always the survivor).
The branch/worktree/PR/project subscriptions that feed the hover
tooltip moved into the tooltip body component, which mounts only while
the tooltip is open — a resting tab now subscribes only to its status
dot's session status and unread count.

packages/ui/src/lib/theme/themes/openchamber-dark.json

@@ -37,7 +37,7 @@
       "border": "#242323",
       "borderHover": "#504e4c",
       "borderFocus": "#da7c47",
-      "selection": "#b9a5992b",
+      "selection": "#c8c6c52b",
       "selectionForeground": "#c9c5ba",
       "focus": "#da7c47",
       "focusRing": "#da7c4755",