docs: Add changelog with versioning (v1.2.0)

- Add comprehensive changelog section following Keep a Changelog format
- Document v1.2.0 changes: SEO agent fixes and z.ai API validation
- Add previous versions (v1.1.0, v1.0.0) for historical context
- Include semantic versioning reference

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

README.md

@@ -53,6 +53,67 @@ Transform vague ideas into production-ready prompts and PRDs. PromptArch is an e
 
 4. Open [http://localhost:3000](http://localhost:3000) to begin.
 
+## 📋 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).
+
+### [1.2.0] - 2025-01-19
+
+#### Added
+- **SEO Agent Behavior Fixes**
+  - SEO agent now stays locked and answers queries through an SEO lens
+  - Smart agent suggestions via `[SUGGEST_AGENT:xxx]` marker for clearly non-SEO tasks
+  - Visual suggestion banner with Switch/Dismiss buttons
+  - Prevented unwanted agent auto-switching mid-response
+- **z.ai API Validation**
+  - Real-time API key validation with 500ms debounce
+  - Inline status indicators:
+    - ✓ Green checkmark with "Validated Xm ago" for valid keys
+    - ✗ Red X with error message for invalid keys
+    - 🔄 Loading spinner during validation
+  - "Test Connection" button for manual re-validation
+  - Persistent validation cache (5 minutes) in localStorage
+  - Clear error messages: "Invalid API key", "Network error", etc.
+- **Store Enhancements**
+  - `ApiValidationStatus` interface for tracking connection states
+  - `apiValidationStatus` state with per-provider tracking
+  - `setApiValidationStatus` action for updating validation results
+
+#### Changed
+- Updated Settings panel with improved UX for API key management
+- Enhanced agent selection behavior to prevent unintended switches
+- Improved error handling for authentication failures
+
+#### Technical Details
+- Files modified:
+  - `lib/store.ts` - Added validation state management
+  - `lib/services/zai-plan.ts` - Added `validateConnection()` method
+  - `lib/services/model-adapter.ts` - Added validation proxy
+  - `components/SettingsPanel.tsx` - Complete rewrite with validation UI
+  - `components/AIAssist.tsx` - Added suggestion handling and UI
+
+### [1.1.0] - 2025-01-15
+
+#### Added
+- GitHub integration for pushing AI-generated artifacts
+- XLSX export functionality for Google Ads campaigns
+- High-fidelity HTML report generation
+- OAuth token management for Qwen API
+
+### [1.0.0] - 2025-01-01
+
+#### Added
+- Initial release of PromptArch
+- Multi-provider AI support (Qwen, Ollama, Z.AI)
+- Prompt Enhancer with 11+ intent patterns
+- PRD Generator with structured output
+- Action Plan generator with framework recommendations
+- Visual canvas for live code rendering
+- Multi-language support (English, Russian, Hebrew)
+
 ## 🛠 Tech Stack
 
 - **Framework**: [Next.js 15.5](https://nextjs.org/) (App Router)