Apply updated Codex repository kit guidance

.codex/project.md

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+# Codex Project Notes
+
+## Project
+
+`Opera-Extensions` currently contains the `Opera Cache Cleaner` extension in `opera-cache-cleaner-extension/`.
+
+Repository:
+
+```text
+Toxic/Opera-Extensions
+```
+
+Gitea URL:
+
+```text
+https://git.wilkensxl.de/Toxic/Opera-Extensions.git
+```
+
+Repository owner and name are derived from this target remote, not from the Codex repository kit remote.
+
+## Commands
+
+Use these commands as the source of truth from the repository root:
+
+```text
+Syntax check background: node --check opera-cache-cleaner-extension/background.js
+Syntax check popup:      node --check opera-cache-cleaner-extension/popup/popup.js
+Build package:           cd opera-cache-cleaner-extension && mkdir -p dist && zip -r dist/opera-cache-cleaner-extension.zip manifest.json background.js popup icons -x "*.DS_Store"
+```
+
+No install, lint, test, README generation, or dependency audit command exists. The extension has no package manifest and no vendored dependencies.
+
+## Stack
+
+```text
+Manifest V3 Chromium extension using plain HTML, CSS, and JavaScript.
+```
+
+Package manager or build tool:
+
+```text
+None.
+```
+
+## Build Artifacts
+
+Release artifacts are produced in:
+
+```text
+opera-cache-cleaner-extension/dist/
+```
+
+Expected file:
+
+```text
+opera-cache-cleaner-extension.zip
+```
+
+## Security Rules
+
+- Keep the extension limited to `browsingData`, `storage`, and `alarms` permissions unless a feature explicitly requires more.
+- The extension must clear only cache data through `chrome.browsingData.remove(..., { cache: true })`.
+- Do not request cookie, history, downloads, tabs, or host permissions without a documented user request.
+- Do not add external network calls.
+- Do not commit secrets, tokens, `.env` files, certificates, or private keys.
+
+## Release Rules
+
+Before a release:
+
+1. run the syntax checks,
+2. review `opera-cache-cleaner-extension/docs/security-review.md`,
+3. update `opera-cache-cleaner-extension/CHANGELOG.md`,
+4. build `opera-cache-cleaner-extension/dist/opera-cache-cleaner-extension.zip`,
+5. load the unpacked extension in Opera for a manual smoke test,
+6. create a tag and release only when the user explicitly asks for it.