add initial changelog tracking releases and updates

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/), and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+---
+
+## [1.1.0] - 2026-06-12
+
+### Added
+* Local offline scanner mode (`--no-ai`) utilizing regex rules for secrets and structure vulnerabilities.
+* CLI-to-Backend sync functionality (`--sync` flag and `sync` command) with associated `/code-scan/sync` API endpoint.
+* Local PDF report generation support (`/export pdf` slash command) built using `fpdf2`.
+* Repository onboarding setup guide (`SETUP.md`).
+* Project contribution guidelines (`CONTRIBUTING.md`).
+* Vulnerability reporting policy (`SECURITY.md`).
+* Pre-commit hook definitions (`.pre-commit-config.yaml`).
+* Developer shortcut workflow automation (`Makefile`).
+* Code of Conduct guidelines (`CODE_OF_CONDUCT.md`).
+
+### Fixed
+* Addressed `aiodns` import and event loop issues during URL scanning.
+
+---
+
+## [1.0.0] - 2026-05-15
+
+### Added
+* Initial release of the SecureLens backend engine.
+* Multi-agent codebase vulnerability triage pipeline supporting Phase 1 (Triage), Phase 2 (Concurrent Analysis), and Phase 3 (Synthesis).
+* Live URL infrastructure auditing across five checks layers (Transport, SSL/TLS, Config headers, Cookies, Path exposure).
+* LiteLLM AI provider client supporting Gemini models.
+* Token-based user authentication (register, login, me profiles).
+* Centralized scan history log.