feat(power): clamshell — ignore lid suspend when docked (#86)

Explicit HandleLidSwitchDocked=ignore under services.logind so closing
the lid with an external display connected does not suspend. Undocked
lid path left to upstream defaults. checks.clamshell-logind asserts the
baked logind.conf and active logind; real multi-monitor lid V3 queued.

Verified: V2 — flake check --no-build; checks.clamshell-logind green.
V3 pending: HARDWARE-QUEUE #86 lid+external on a laptop.

README.md

@@ -284,7 +284,7 @@ option, e.g. `services.easyeffects.enable = lib.mkForce false;` — see
 | `nomarchy.system.snapper.enable` | `false` | Hourly/daily BTRFS timeline snapshots + `nixos-rebuild-snap` (installer enables it; no-op unless root is BTRFS) |
 | `nomarchy.system.power.enable` | `true` | Active power management (see below) |
 | `nomarchy.system.power.backend` | `"ppd"` | `"ppd"` (power-profiles-daemon + menu/Waybar switcher) or `"tlp"` (deeper battery tuning, no switcher) — mutually exclusive |
-| `nomarchy.system.power.laptop` | `false` | Marks a laptop, gating battery-only features; the installer sets it when a battery is present |
+| `nomarchy.system.power.laptop` | `false` | Marks a laptop, gating battery-only features; the installer sets it when a battery is present. With power management on, clamshell is also configured: lid close ignores suspend when logind considers the machine docked (external display) — `HandleLidSwitchDocked=ignore` |
 | `nomarchy.system.power.thermal.enable` | `false` | thermald (Intel-only); the installer enables it on a GenuineIntel CPU |
 | `nomarchy.system.power.batteryChargeLimit` | `null` | Stop charging at this % (e.g. `80`) where the hardware supports it; needs `power.laptop` |
 | `nomarchy.hardware.intel.enable` | `false` | Intel enablement above nixos-hardware (GuC/HuC firmware via `i915.enable_guc=3` — `.guc` toggles just that; the installer unsets it on `xe`-driver GPUs); the installer sets it on an Intel CPU/GPU |