feat: OpenCode Arka-AI installer and daily chat catalog

Public one-liner for opencode. Catalog is the filtered chat list from
api.arka-ai.ru/v1/models, refreshed daily at 05:00 Europe/Moscow.

README.md

@@ -0,0 +1,48 @@
+# Arka public artifacts
+
+Public, anonymous files for Arka AI. Private product code stays in other Gitea
+repos.
+
+## OpenCode Arka-AI provider
+
+`arka-ai.ru` is not in models.dev. One command installs the OpenCode provider
+plugin, companion skill, and (optional) API key:
+
+```bash
+curl -fsSL https://git.arka-ai.ru/arka/public/raw/branch/main/install-arka-provider.sh | bash
+```
+
+With a key and a default model:
+
+```bash
+curl -fsSL https://git.arka-ai.ru/arka/public/raw/branch/main/install-arka-provider.sh \
+  | bash -s -- --key "$ARKA_API_KEY" --set-default DeepSeek-V4-Pro
+```
+
+Then open a **new shell** and restart opencode. `/models` should list **Arka-AI**
+(~116 chat models).
+
+The installer is offline and idempotent. It does not call `api.arka-ai.ru`.
+`python3` is required only with `--set-default`.
+
+### Files
+
+| Path | Role |
+|------|------|
+| `install-arka-provider.sh` | Self-extracting installer (the one-liner) |
+| `plugins/arka-ai.ts` | Provider plugin source (embedded in the installer) |
+| `skills/arka-provider/SKILL.md` | Companion skill source (embedded in the installer) |
+| `catalog.json` | Filtered chat catalog, refreshed daily at **05:00 Europe/Moscow** from `https://api.arka-ai.ru/v1/models` |
+
+Forced keep-list: `DeepSeek-V4-Pro`, `deepseek-v4-flash-0731`, `qwen3.8-max`,
+`kimi-k3`, `MiniMax-M3`, `GLM-5.3`.
+
+Inference uses `ARKA_API_KEY` against `https://api.arka-ai.ru/v1`. Listing the
+catalog does not require a key.
+
+After install:
+
+1. New shell so `ARKA_API_KEY` is exported (GUI/desktop launches may need the
+   key set in that session, not only `~/.bashrc`).
+2. Restart opencode, run `/models`.
+3. Refresh later: `rm -f ~/.cache/opencode/arka-ai-models.json` and restart.