Refine architecture and design philosophy documentation

docs/DESIGN-PHILOSOPHY.md

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+# ChronoSeal Design Philosophy
+
+**"Files as a Software" (FaaS) — Everything is a file.**
+
+ChronoSeal is intentionally designed as a **first-class citizen of Linux**. The entire application behaves as if it is a well-designed native file in the Unix filesystem.
+
+### Core Beliefs
+
+- The CLI is the single source of truth.
+- The application must be controllable, inspectable, configurable, and composable using standard Unix tools.
+- Any GUI, TUI, or web interface is only a thin wrapper.
+- Production robustness and decades-long maintainability take precedence over rapid development.
+
+### Key Principles Applied
+
+- **Behave like a file**: Clear interface, predictable behavior, proper lifecycle (open/read/write/close semantics via signals and commands).
+- **Composability**: Works naturally with pipes, redirection, systemd, scripts, and orchestration tools.
+- **Observability**: Everything important is exposed as text or structured data.
+- **Minimal Friction**: One-line installer, excellent `--help`, proper man pages.
+- **Respect for the OS**: Follows FHS, XDG, systemd best practices, and hardened security model.
+
+This philosophy guided the complete refactoring of ChronoSeal.
+
+**Status**: Core architecture and systemd integration completed. Rich CLI, configuration system, and installer are in progress.