docs: add project planning documents

docs/adrs/adr-001-tech-stack.md

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+# ADR-001: Go Backend with Preact Frontend
+
+## Status
+Accepted
+
+## Context
+We need to choose a backend language/framework and frontend technology for a documentation platform with these requirements:
+- Ultra-fast performance
+- Minimal external dependencies
+- No vendor lock-in
+- Battle-tested security
+- Server-side rendering for non-interactive pages
+- Offline-capable web client
+
+## Decision
+**Backend**: Go 1.22+ with standard library plus minimal, well-audited packages  
+**Frontend**: Preact 10 with TypeScript, Vite build system
+
+## Consequences
+
+### Positive
+- Go's standard library is comprehensive and security-audited
+- Single static binary deployment — no runtime dependencies
+- Preact is 10KB (vs 40KB+ React) with identical API
+- Go's `html/template` provides safe SSR with auto-escaping
+- TypeScript gives type safety without runtime overhead
+- Both ecosystems have excellent supply-chain security (Go modules with checksums, npm audit)
+
+### Negative
+- Go template syntax is less expressive than JSX
+- WebSocket sync logic must be implemented manually (no framework like Socket.io)
+- Preact ecosystem smaller than React (mitigated by using mostly native APIs)
+
+## Alternatives Considered
+
+### Rust + Axum
+- **Pros**: Memory safety guarantees, excellent performance
+- **Cons**: Longer compile times, smaller talent pool, steeper learning curve for team
+- **Rejected**: Go's pragmatic balance of safety and velocity better fits project timeline
+
+### Deno Fresh
+- **Pros**: Island architecture, native TypeScript, no build step
+- **Cons**: Newer ecosystem, fewer audited libraries, Deno runtime less battle-tested than Go
+- **Rejected**: Go's maturity and deployment simplicity preferred
+
+### Next.js / React Server Components
+- **Pros**: Mature ecosystem, built-in SSR
+- **Cons**: Heavy bundle size, complex build system, RSC lock-in, many dependencies
+- **Rejected**: Violates "minimal dependencies" and "no vendor lock-in" principles
+
+## References
+- Go Security Policy: https://go.dev/security
+- Preact Size Comparison: https://bundlephobia.com/package/preact@10.19.3