chore: initialize repository with project docs and tooling

- Add persistent docs: PRD, functional design, architecture, repository structure, development guidelines, milestones

- Add brainstorming notes (docs/ideas/)

- Configure tooling: package.json, tsconfig, eslint, prettier, vitest

- Add opencode configuration (commands, skills, agents)

- Add .gitignore for node_modules, .env, data/, backups/, build outputs

opencode.json

@@ -0,0 +1,7 @@
+{
+  "$schema": "https://opencode.ai/config.json",
+  "instructions": [
+    "AGENTS.md"
+  ],
+  "permission": "allow"
+}