Add comprehensive README with architecture overview, security model, and setup guides

Document the complete pwr project covering the three-crate architecture,
security model with its three layers (TLS transport, PSK authentication,
and age at-rest encryption), quick-start guides for both Debian 12 server
and Arch Linux client, the .project.toml file format specification,
complete command reference table, configuration file reference for both
client and server TOML files, and build-from-source instructions with
feature flags for the optional TUI.