docs: rename project to gacha-codes-notifier, link @exec-astraea

The README still called the project hoyo-codes, which no longer matches
the repository. Renames the heading and the Docker image/container names
in the run example, and makes the @exec-astraea credit an actual link.

References to seriaati/hoyo-codes are left alone — that is a different
project that genuinely has that name.

Also updates the User-Agent sent to the upstream APIs: it advertised
github.com/exec-astraea/hoyo-codes, which 404s (no redirect). The whole
point of the +URL is letting API operators identify the client, so a dead
link defeats it.

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

Author: exec-astraea

README.md

@@ -1,4 +1,4 @@
-# hoyo-codes
+# gacha-codes-notifier
 
 Watches for new **gacha redemption codes** (Genshin Impact, Honkai: Star Rail,
 Zenless Zone Zero, Honkai Impact 3rd, Wuthering Waves, and Arknights: Endfield)
@@ -13,12 +13,12 @@ and Arknights: Endfield. See [Sources](#sources).
 ```sh
 cp config.example.yaml config.yaml    # then add your webhook(s) + messages
 
-docker build -t hoyo-codes .
-docker run -d --name hoyo-codes \
+docker build -t gacha-codes-notifier .
+docker run -d --name gacha-codes-notifier \
   -e TZ=Europe/Amsterdam \
   -v "$PWD/config.yaml:/app/config.yaml:ro" \
   -v "$PWD/data:/app/data" \
-  hoyo-codes
+  gacha-codes-notifier
 ```
 
 Or without Docker: `CONFIG_FILE=config.yaml go run .`
@@ -55,4 +55,4 @@ belongs to others:
 
 ## License
 
-[AGPL-3.0](LICENSE) © Alisa Frelia @exec-astraea.
+[AGPL-3.0](LICENSE) © Alisa Frelia [@exec-astraea](https://github.com/exec-astraea).

sources.go

@@ -18,7 +18,7 @@ type Code struct {
 
 var httpClient = &http.Client{Timeout: 20 * time.Second}
 
-const userAgent = "hoyo-codes-notifier/1.0 (+https://github.com/exec-astraea/hoyo-codes)"
+const userAgent = "gacha-codes-notifier/1.0 (+https://github.com/exec-astraea/gacha-codes-notifier)"
 
 func getJSON(url string, out any) error {
 	req, err := http.NewRequest(http.MethodGet, url, nil)