docs: v1.6.0 Release Documentation - Comprehensive Changelog & README Updates (#184)

* docs: v1.6.0 release documentation - comprehensive changelog and README updates

- Update CHANGELOG.md with comprehensive v1.6.0 release notes covering 20+ PRs
  - PostgreSQL Extensions: LATERAL JOIN, JSON/JSONB operators, DISTINCT ON, FILTER clause
  - Language Server Protocol: Full LSP implementation with diagnostics, completion, hover
  - VSCode Extension: Official extension with syntax highlighting and formatting
  - Token Type Unification: 14x faster token comparison with O(1) int-based lookups
  - Linter Rules: 10 built-in rules (L001-L010) with auto-fix capabilities
  - Security Scanner: Enhanced SQL injection detection
  - Developer Tools: go-task Taskfile.yml, structured error codes (E1001-E3004)
  - Performance: 575x faster keyword suggestions with caching
- Update README.md with v1.6.0 highlights and "What's New" section
- Update version to 1.6.0 in root.go and doc.go
- Add footer links for v1.6.0 in CHANGELOG

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Co-Authored-By: Claude Opus 4.5 <[email protected]>

* fix: update rootCmd.Version to 1.6.0 for consistency

Address PR review feedback - the rootCmd.Version field was still set to
"1.4.0" while the Version variable was correctly updated to "1.6.0".

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

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Co-authored-by: Ajit Pratap Singh <[email protected]>
Co-authored-by: Claude Opus 4.5 <[email protected]>

Author: 3 people