Add fork documentation and rewrite README

- README: watch-first feature overview, install + libmpv requirement,
  fork attribution.
- docs/UPSTREAM.md: upstream merge procedure, expected conflict zones,
  list of SageTube-only modules.
- docs/BUILDING.md: per-OS libmpv install, managed-binary locations,
  playback reliability notes, warning that inherited GitHub release
  workflows download appimagetool/ffmpeg without checksum verification.

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

README.md

@@ -1,12 +1,45 @@
 # SageTube
 
-A watch-first YouTube client for the desktop — search, browse channels and playlists, follow subscriptions, and stream videos in an embedded mpv player, with full yt-dlp download capability inherited from [YTSage](https://github.com/oop7/YTSage).
+A watch-first YouTube client for the desktop. Search, browse channels and
+playlists, follow subscriptions, and stream videos in an embedded mpv player —
+with the full yt-dlp download feature set inherited from
+[YTSage](https://github.com/oop7/YTSage).
 
-SageTube is a fork of [YTSage](https://github.com/oop7/YTSage) by [oop7](https://github.com/oop7) (MIT). The original YTSage documentation is preserved at [docs/UPSTREAM_README.md](docs/UPSTREAM_README.md).
+## Features
 
-## Status
+**Watching**
+- 📺 Embedded mpv player (Wayland/X11/Windows/macOS) with quality selector,
+  speed, volume, subtitles, fullscreen
+- 🔍 In-app YouTube search with thumbnail card grid
+- 📂 Channel browsing (Videos / Shorts / Live) and playlist views
+- 🔔 Local subscriptions — no Google account needed — with an aggregated feed;
+  optional real account feed via browser cookies
+- ⏯️ Watch history with resume positions and a persistent play queue
+- ⬇️ One click from any video card into the downloader
 
-Under active development. See the upstream README for the downloader feature set, which remains fully functional.
+**Downloading** (from YTSage)
+- Format table, audio extraction, subtitles, SponsorBlock, chapters,
+  playlists, trimming, speed limits, proxies, cookies, custom yt-dlp commands,
+  download history — see the [upstream README](docs/UPSTREAM_README.md)
+
+## Install
+
+```bash
+pip install .
+sagetube
+```
+
+Requires **libmpv** for playback (Arch: `pacman -S mpv`) — see
+[docs/BUILDING.md](docs/BUILDING.md). Without it the app works as a downloader.
+
+## Fork notes
+
+SageTube is a fork of [YTSage](https://github.com/oop7/YTSage) by
+[oop7](https://github.com/oop7) (MIT). The internal package keeps the
+`ytsage` name so upstream fixes remain mergeable — see
+[docs/UPSTREAM.md](docs/UPSTREAM.md). The fork also fixes a number of
+upstream bugs (unsafe partial-file cleanup, unverified binary updates,
+thread-safety issues, PATH corruption on Windows — see the git log).
 
 ## License