feat(waybar): explain 'Not charging' hold state in the battery tooltip

With a charge cap, AC-plugged inside the hysteresis band (power.nix:
start = cap − 10) reports 'Not charging' — indistinguishable from a
broken charger without a hint. tooltip-format-plugged now says the cap
is holding the level and how charging resumes. No cap value baked: the
menu changes it live without a rebuild.

Whole-swap parity: the same tooltip added to all four theme
waybar.jsonc files (boreal/executive-slate/summer-day/summer-night) —
the generated config is bypassed under these themes.

Verified on hardware (TuringMachine, boreal theme, cap 80, battery 71%
in the hold band): all four jsonc validate with jq; home-manager switch
deploys the boreal config with the tooltip; battery currently in the
exact Plugged state the tooltip targets.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

themes/executive-slate/waybar.jsonc

@@ -118,6 +118,7 @@
     "format": "<span size='13pt'>{icon}</span>{capacity}%",
     "format-charging": "<span size='13pt'>  </span>{capacity}%",
     "format-plugged": "<span size='13pt'>  </span>{capacity}%",
+    "tooltip-format-plugged": "Plugged in, not charging — held at {capacity}% by the battery charge cap\nCharging resumes ~10% below the cap · click to adjust",
     "format-icons": [
       "  ",
       "  ",