docs: add opencode-zen-free preset doc (#690)

Mirror the $30 preset doc style: intro, config block, 3-column skill
reference, and a Configuration link. Add it to the README Preset Docs list.

README.md

@@ -172,6 +172,7 @@ The default generated configuration includes both `openai` and `opencode-go` pre
 - **[OpenCode Go Preset](docs/opencode-go-preset.md)** — runs the agents on OpenCode Go models; enables the Observer agent for visual analysis since its orchestrator model isn't multimodal.
 - **[Author's Preset](docs/authors-preset.md)** — the exact config the author runs day to day, with third-party skills.
 - **[$30 Preset](docs/thirty-dollars-preset.md)** — a mixed-provider setup built around Codex Plus and GitHub Copilot Pro for about $30/month.
+- **[OpenCode Zen Free Preset](docs/opencode-zen-free-preset.md)** — a fully free-tier preset; every agent runs on an \`opencode/...-free\` model, so it costs nothing.
 
 ### For Alternative Providers
 

docs/opencode-zen-free-preset.md

@@ -0,0 +1,75 @@
+# OpenCode Zen Free Preset
+
+This preset is a **fully free-tier** setup — every agent uses an `opencode/...-free`
+model, so it costs nothing to run.
+
+No subscriptions, no API keys, no billing. Just configure the preset and go.
+
+---
+
+## The Config
+
+```jsonc
+{
+  "preset": "opencode-zen-free",
+  "presets": {
+    "opencode-zen-free": {
+      "orchestrator": {
+        "model": "opencode/hy3-free",
+        "temperature": 0.4,
+        "skills": ["*"],
+        "mcps": ["*", "!context7"]
+      },
+      "oracle": {
+        "model": "opencode/big-pickle",
+        "temperature": 0.4,
+        "variant": "max",
+        "skills": ["simplify"],
+        "mcps": []
+      },
+      "explorer": {
+        "model": "opencode/north-mini-code-free",
+        "temperature": 0.2,
+        "skills": [],
+        "mcps": []
+      },
+      "librarian": {
+        "model": "opencode/deepseek-v4-flash-free",
+        "temperature": 0.2,
+        "skills": [],
+        "mcps": ["websearch", "context7", "gh_grep"]
+      },
+      "designer": {
+        "model": "opencode/mimo-v2.5-free",
+        "temperature": 0.3,
+        "variant": "medium",
+        "skills": [],
+        "mcps": []
+      },
+      "fixer": {
+        "model": "opencode/deepseek-v4-flash-free",
+        "temperature": 0.2,
+        "variant": "high",
+        "skills": [],
+        "mcps": []
+      },
+      "observer": {
+        "model": "opencode/north-mini-code-free",
+        "temperature": 0.2,
+        "variant": "low",
+        "skills": [],
+        "mcps": []
+      }
+    }
+  }
+}
+```
+
+## Skill Reference
+
+| Skill | Description | Source |
+| --- | --- | --- |
+| `*` | All installed skills (wildcard) | public |
+| `simplify` | Code simplification | public |
+
+For the complete configuration reference, see [Configuration](configuration.md).