docs: add conventional commits guide and design decisions template

DESIGN.md

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+# FrogFPS — Design Decisions Log
+
+*Track important design choices here. Each entry explains what was decided, why, and any alternatives considered.*
+
+---
+
+## Template
+
+```markdown
+### [Date] — [Feature/Decision Name]
+
+**Decision:** What was chosen
+
+**Rationale:** Why this choice was made
+
+**Alternatives Considered:**
+- Option A: ...
+- Option B: ...
+
+**Trade-offs:**
+- Pros: ...
+- Cons: ...
+
+**Related Files:**
+- `path/to/file.cs`
+- `path/to/scene.tscn`
+```
+
+---
+
+## Entries
+
+<!-- Add entries below this line -->