feat: add whyoolw/sharednd — auto DND while screen sharing (niri)

Headless service that follows niri msg -j event-stream and enables
notification Do Not Disturb via the host IPC (notification-dnd-set)
while at least one screencast is active, restoring it afterwards.

Side effects: spawns a persistent shell retry loop around
`niri msg -j event-stream`, runs `niri msg -j casts` /
`niri msg version` and `noctalia msg notification-dnd-*` on demand.
No network access, no filesystem writes.

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

sharednd/README.md

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+# ShareDND
+
+Automatic Do Not Disturb while the screen is being shared.
+
+When the first screencast starts (any app sharing via the portal — Discord,
+OBS, browsers, `niri msg action set-dynamic-cast-*`, ...), notification DND is
+enabled. When the last screencast stops, notifications come back.
+
+## How it works
+
+- A headless service follows `niri msg -j event-stream` and reacts to
+  `CastsChanged` / `CastStartedOrChanged` / `CastStopped` events — no polling.
+- On every cast event it re-queries `niri msg -j casts` as the authoritative
+  state, so missed or reordered events cannot desync it. The stream is wrapped
+  in a shell retry loop and resends full state on reconnect.
+- DND is toggled through the host IPC (`noctalia msg notification-dnd-set`),
+  so the usual OSD feedback appears.
+
+## Ownership rules
+
+- If DND was **already on** before sharing started, the plugin leaves it on
+  after sharing ends (it never took ownership). The
+  "Always disable DND after sharing" setting overrides this.
+- If DND was enabled manually *during* sharing while the plugin owned it, it
+  will still be turned off when sharing ends (the plugin cannot tell the
+  difference).
+
+## Settings
+
+- **Only active streams** — count only casts with `is_active: true`. Off by
+  default: any open screencast session (even paused or showing nothing) keeps
+  DND on.
+- **Always disable DND after sharing** — force DND off when sharing ends,
+  regardless of its state before sharing started.
+
+## Requirements & limitations
+
+- Requires **niri** (detection is niri IPC; on other compositors the plugin
+  is inert and shows a warning at startup).
+- There is no plugin-disable hook in the host: if you disable the plugin
+  mid-share while it owns DND, DND stays on — toggle it manually.