docs: v2.0.0 changelog, configuration for auth/boards, announcement refresh

- CHANGELOG 2.0.0 entry covering all Phase 5 waves
- configuration.md: transports, per-monitor alert rules, auth, boards,
  appearance/language, PWA
- announcement.md: updated for shipped features and opt-in auth
- AGENTS.md: "no auth" line was stale
- ROADMAP: tick shipped M1–M3 planning checkboxes

Authored By: TDvorak <[email protected]>

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

AGENTS.md

@@ -1,7 +1,8 @@
 # Dash
 
 Self-hosted homelab dashboard. Single Go binary serving an embedded React UI.
-No auth, no YAML. See `ROADMAP.md` (canonical) and `docs/specs/` for design.
+Optional auth (off by default), no YAML. See `ROADMAP.md` (canonical) and
+`docs/specs/` for design.
 
 ## Layout