docs(signals): rewrite testing guide (#5107)

Closes #5106

Author: rainerhahnekamp

.agents/skills/writing-docs/SKILL.md

@@ -0,0 +1,29 @@
+---
+name: writing-docs
+description: Doc conventions (impersonal style, formatting, Vitest in examples, structure, linking). Use when writing, editing, or reviewing guides, tutorials, or READMEs.
+---
+
+# Writing Documentation
+
+Apply when writing, editing, or reviewing docs.
+
+## Voice and style
+
+- **Impersonal:** No "you", "your", or second-person imperatives.
+- **Statements over imperatives:** Prefer "Tests should not spy" over "Avoid spying"; passive or descriptive phrasing.
+- **Titles:** Gerunds, not imperatives (e.g. "Testing the store" not "Test the store").
+- **Tone:** Direct and minimal; every sentence adds information.
+
+## Formatting
+
+- **Dashes:** Space-hyphen-space (`-`), not em dash (`—`).
+- **Paragraphs:** Merge single-sentence paragraphs when it helps readability.
+
+## Code examples
+
+- **Examples:** Simple, self-contained (e.g. Counter, Books). Short snippets; shorten or link out long async flows unless the topic is async.
+
+## Structure and navigation
+
+- **Section titles:** Accurate and scannable.
+- **Links over repetition:** Link to other guides; explain only what is specific to this page.