docs: add architecture diagrams

Author: glitch418x

docs/ARCHITECTURE.md

@@ -0,0 +1,51 @@
+# Architecture
+
+Local host health-check daemon. Runs checks, tracks state, sends alerts on transitions.
+
+## System Context
+
+```mermaid
+graph LR
+  Cron[LaunchAgent] -->|interval| Daemon
+  Daemon -->|shell exec| Checks[Health Checks]
+  Daemon -->|on transition| Notify[Notifiers]
+  Notify --> Ntfy[(ntfy)]
+  Notify --> Cmd[(command)]
+  CLI[CLI] -->|one-shot| Checks
+  CLI -->|TUI| Status[Status View]
+  Status --> Checks
+  Daemon & CLI -->|read| Config[(~/.config/config.toml)]
+  Daemon -->|write| Alerts[(alerts.log)]
+```
+
+## Components
+
+| Component | Responsibility |
+|-----------|---------------|
+| **cmd** | Cobra CLI: check, status, daemon, init, validate, notify |
+| **runner** | Execute checks, filter, collect results |
+| **daemon** | Continuous loop, fail/recover transition tracking |
+| **notify** | Alert backends (ntfy, command), cooldown logic |
+| **tui** | Bubbletea status display, watch mode |
+| **config** | TOML parsing, validation |
+
+## Check Lifecycle
+
+```mermaid
+stateDiagram-v2
+  [*] --> Passing: check passes
+  [*] --> Failing: check fails
+  Passing --> Failing: check fails → alert CRIT
+  Failing --> Passing: check passes → alert RECOVERED
+  Failing --> Failing: still failing (cooldown suppresses)
+```
+
+## Key Design Decisions
+
+| Decision | Choice | Why |
+|----------|--------|-----|
+| Shell-exec checks | Run any command | Maximum flexibility, no custom check API |
+| Detect only (v1) | No auto-remediation | Keep it simple, alert humans |
+| Transition alerts | Only on state change | No alert spam |
+| Cooldown per backend | Configurable per notifier | Different urgency per channel |
+| Status TUI bypasses bubbletea in non-watch | Direct View() render | Works without TTY (CI, pipes) |