feat: make the terminal UI the default - #111

Summary

• make bare kb launch the TUI only on interactive stdin and stdout; non-interactive invocation prints help without opening data
• move the hardened API-only server to explicit kb serve, preserving every API/auth/import/forge contract while returning 404 for all other paths and methods
• delete the React/Vite/npm/dist/Sonar surface and make CI/release Go-only
• rewrite operator documentation for TUI-first use, optional API service migration, and unchanged SQLite data

Verification

• checker self-test, exact Go coverage (96.4%), full race, vet, format, CGO-disabled build
• CI monitor built-ins test and action pin check
• ShellCheck and release script syntax
• clean plain-clone release dry-run with unchanged HEAD/tree/index and all five cross-builds
• binary TTY/non-TTY, CLI, MCP, serve, API 404/security, remote CLI, and version matrix
• parallel Spec and Standards reviews: no findings

Closes #96