docs: update greet() description to match current behaviour

## Docs drift detected

`docs/overview.md` was out of sync with `src/hello.js`.

### What changed

| File | Before | After |
|------|--------|-------|
| `docs/overview.md` | "returns a greeting string" | Documents the actual return format and default parameter |

### Details

`src/hello.js` exports:
```js
export function greet(name = "world") {
return `Bonjour, ${name}!`;
}
```

The docs did not describe:
- The default parameter value (`"world"`)
- The actual return format (`` `Bonjour, ${name}!` ``)
- Usage examples

This PR updates `docs/overview.md` with accurate, minimal documentation matching the current implementation.

---
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---

> [!NOTE]
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> The changes have been pushed to branch `docs/fix-greet-description-bde8bad37579cbff`.
>
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Show patch (37 lines)

```diff
From 4aad32efc12b3adbdb6f5ff7acaa77c329be906e Mon Sep 17 00:00:00 2001
From: "github-actions[bot]"
Date: Mon, 3 Aug 2026 06:52:14 +0000
Subject: [PATCH] docs: update greet() description to match current behaviour

- Document default parameter name = "world"
- Describe actual return format: Bonjour, ${name}!
- Add usage examples

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
docs/overview.md | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/docs/overview.md b/docs/overview.md
index baa0f7d..d960ace 100644
--- a/docs/overview.md
+++ b/docs/overview.md
@@ -1,5 +1,12 @@
# Overview

-`greet(name)` returns a greeting string.
+`greet(name = "world")` returns a French greeting string in the format `Bonjour, ${name}!`.

-(Intentionally simple — change `src/hello.js` without updating this file to exercise docs drift.)
+**Example:**
+
+```js
+import { greet } from "./src/hello.js";
+
+greet(); // "Bonjour, world!"
+greet("Alice"); // "Bonjour, Alice!"
+```
--
2.54.0

```

Read docs/overview.md alongside src/hello.js to compare the documented greet() behavior with the current implementation. Confirm the default parameter, return format, and usage examples are accurately documented, then verify the documentation matches the shown behavior.

Written by the indexing model from the issue text.