docs: add CLAUDE.md project instructions

Co-Authored-By: Virgil <virgil@lethean.io>

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+PHP library (`lthn/php-plug-social`) providing social media provider integrations for the Plug framework. Licensed under EUPL-1.2. Requires PHP 8.2+ and depends on `lthn/php` (the core Plug framework).
+
+## Commands
+
+```bash
+composer install          # Install dependencies
+```
+
+No tests, linter, or build steps exist in this repository yet.
+
+## Architecture
+
+**Namespace:** `Core\Plug\Social\` (PSR-4 autoloaded from `src/`)
+
+### Provider Structure
+
+Each social provider lives in its own subdirectory under `src/` and follows a consistent class pattern:
+
+| Class | Contract | Purpose |
+|-------|----------|---------|
+| `Auth` | `Authenticable` (+ `Refreshable` if supported) | OAuth2 flow: auth URL, token exchange, account info |
+| `Post` | `Postable` | Publishing content via `publish(string $text, Collection $media, array $params)` |
+| `Read` | `Readable` | Fetching content via `get(string $id)` and `list(array $params)` |
+| `Delete` | `Deletable` | Removing content via `delete(string $id)` |
+| `Media` | `MediaUploadable` | Uploading media via `upload(array $item)` |
+
+Some providers have extra classes: `Pages` (Meta, LinkedIn), `Boards` (Pinterest), `Groups` (VK), `Subreddits` (Reddit), `Comment` (YouTube).
+
+### Providers
+
+Twitter, Meta (Facebook/Instagram), LinkedIn, Pinterest, Reddit, TikTok, VK, YouTube.
+
+### Shared Traits and Contracts (from `lthn/php`)
+
+All provider classes compose three traits from the core framework:
+
+- **`BuildsResponse`** — wraps API responses into `Core\Plug\Response` objects via `fromHttp()` and `error()`
+- **`ManagesTokens`** — provides `withToken()`, `accessToken()`, `getToken()` for OAuth token handling
+- **`UsesHttp`** — provides `http()` (returns Laravel HTTP client) and `buildUrl()` for URL construction
+
+Contracts (`Authenticable`, `Refreshable`, `Postable`, `Readable`, `Deletable`, `MediaUploadable`) define the interface each class implements.
+
+### Conventions
+
+- All classes use `declare(strict_types=1)`
+- API base URLs are stored as `private const API_URL`
+- Auth classes have static `identifier()` (slug) and `name()` (display name) methods
+- Post/Auth classes provide `externalPostUrl()` / `externalAccountUrl()` static helpers for building user-facing URLs
+- Token expiry is stored as a UTC timestamp via `Carbon::now('UTC')->addSeconds()`
+- Media upload happens inline during `Post::publish()` — the Post class instantiates Media, uploads each item, collects IDs, then publishes
+- Provider-specific headers (e.g. LinkedIn's `X-Restli-Protocol-Version`, Reddit's `User-Agent`) are defined in `httpHeaders()` / `redditHeaders()` methods