🤖 Fix proofreading issues in bevy skills

- Fix `ref` keyword used as variable name in code examples
- Fix "retries next tick" → systems continue on normal schedule
- Fix `State::set()` → `NextState::set()` in upgrade skill
- Fix `SceneRoot` → `WorldAssetRoot` for 0.19 consistency
- Fix Replace lifecycle event description (overwritten, not removed)
- Fix grep patterns (quoting, trailing spaces)
- Remove third-party physics/networking content (not Bevy built-ins)

plugins/bevy/skills/bevy-0.19/references/events-observers.md

@@ -72,7 +72,7 @@ Built-in events observers can watch per component:
 |-------|-------|-------|
 | `Add` | component added where absent | first |
 | `Insert` | any insert (add or replace) | after `Add` |
-| `Replace` | component removed, regardless of replacement | before `Remove` |
+| `Replace` | component value overwritten by a new insert over an existing value | before `Remove` |
 | `Remove` | component removed | after `Replace` |
 | `Despawn` | entity despawned (per component) | last |