Step 9: Polish — build system, error messages, goreleaser, demo GIF

## Context

Final polish before HN launch. Everything should feel smooth.

## Scope

### Build system
- Makefile with build, test, lint targets
- Cross-platform build (linux/amd64, linux/arm64, darwin/arm64)
- goreleaser config for GitHub releases
- Version injection via ldflags (`-X main.version=...`)

### UX polish
- Error messages that don't suck — human-readable, actionable
- "No API key found" → "Set GEMINI_API_KEY or add api_key_env to yantra.toml"
- "provider not found" → "Provider 'foo' not in config. Available: openai, anthropic, gemini"
- `yantra init` auto-detects available API keys from environment
- `yantra tui` auto-starts daemon if not running
- Clean `--help` output for every command

### Demo
- README with animated GIF showing the full flow
- Example configs for common setups
- `go install` one-liner in README

### Launch checklist (from mvp.md)
- [ ] `go install` → `yantra tui` works in under 30 seconds
- [ ] Streams tokens visually in TUI
- [ ] Can read/write files
- [ ] Can search the web
- [ ] Memory persists across sessions
- [ ] MCP servers work (demo with filesystem or GitHub MCP)
- [ ] Works with OpenAI, Anthropic, and Gemini
- [ ] Single binary, no Docker required
- [ ] README has animated GIF
- [ ] Clean --help output