docs: update greet() docs to reflect actual implementation

## Summary

`docs/overview.md` was behind the current `src/hello.js` implementation.

### What drifted

| Item | Code (`src/hello.js`) | Docs (`docs/overview.md`) before this PR |
|---|---|---|
| Default parameter | `name = "world"` | Not mentioned |
| Return format | `` `Bonjour, ${name}!` `` (French greeting) | "returns a greeting string" |
| Usage examples | — | None |

### Changes

- Added function signature with default parameter
- Documented the actual return value format
- Added two usage examples

### Reviewer notes

This is a minimal, accurate update. No application code was changed.

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---

> [!NOTE]
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> The changes have been pushed to branch `docs/fix-greet-overview-cdb4a1bb7c9ce8ca`.
>
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Show patch (39 lines)

```diff
From f383c4f892c0a2cceeab4f2d3dd591ae92f83880 Mon Sep 17 00:00:00 2001
From: "github-actions[bot]"
Date: Mon, 20 Jul 2026 06:49:31 +0000
Subject: [PATCH] docs: update greet() docs to match current implementation

- Add default parameter (name = "world")
- Document the French greeting format ("Bonjour, ${name}!")
- Add usage examples

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
docs/overview.md | 12 ++++++++++++
1 file changed, 12 insertions(+)

diff --git a/docs/overview.md b/docs/overview.md
index baa0f7d..e78e6bd 100644
--- a/docs/overview.md
+++ b/docs/overview.md
@@ -2,4 +2,16 @@

`greet(name)` returns a greeting string.

+**Signature:** `greet(name = "world")`
+
+**Returns:** `` `Bonjour, ${name}!` `` — a French-style greeting with the provided name.
+
+**Example:**
+```js
+import { greet } from "./src/hello.js";
+
+greet(); // "Bonjour, world!"
+greet("Alice"); // "Bonjour, Alice!"
+```
+
(Intentionally simple — change `src/hello.js` without updating this file to exercise docs drift.)
--
2.54.0

```

Read src/hello.js to confirm the current greet() signature and return format, then compare it with docs/overview.md. Done means the documentation describes the default parameter, French greeting output, and both usage examples accurately; the issue body indicates these changes already exist on a pushed branch.

Written by the indexing model from the issue text.