docs: add CHANGELOG.md with release history

CHANGELOG.md

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+# Changelog
+
+## [2026.05.2] — 2026-05-23
+
+### Added
+- Comprehensive test suite (94 tests across monitor, server, cluster)
+- golangci-lint config with CI enforcement
+- Gitea Actions CI pipeline (test + lint)
+- Graceful shutdown for HTTP and SSH servers
+- Context-aware alert delivery with timeout
+- Request size limits on all POST endpoints
+- Constant-time secret comparison
+- Check interval jitter to prevent thundering herd
+- `--version` flag with build metadata injection
+
+### Fixed
+- Silent JSON unmarshal failures in alert settings
+- Panic on crypto/rand failure replaced with error return
+- Alert delivery errors now logged instead of swallowed
+- log.Fatalf in goroutines replaced with log.Printf
+- Deprecated LineUp/LineDown API calls
+
+### Security
+- Cluster secret compared with crypto/subtle (timing-safe)
+- http.MaxBytesReader on all JSON endpoints
+- ReadHeaderTimeout added to HTTP server
+
+## [2026.05.1] — 2026-05-14
+
+### Added
+- Distributed probing with leader + probe nodes
+- Config-as-code (YAML apply/export with dry-run, prune)
+- TUI visual polish (zebra striping, sparklines, breadcrumbs)
+- Incident management and maintenance windows
+- 9 alert providers (Discord, Slack, Email, Ntfy, Telegram, PagerDuty, Pushover, Gotify, Webhook)
+
+## [2026.04.1] — Initial independent fork
+
+### Added
+- SSH-accessible TUI (Bubble Tea + Wish)
+- 6 check types (HTTP, Push, Ping, Port, DNS, Group)
+- SQLite and PostgreSQL support
+- HA clustering with automatic failover
+- Prometheus metrics endpoint
+- Public status page
+- Uptime Kuma import