docs: add changelog conventions

Author: hanselhansel

docs/changelog-conventions.md

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+# Changelog Conventions
+
+Context CLI follows the [Keep a Changelog](https://keepachangelog.com/en/1.1.0/) format for documenting notable changes.
+
+## Format
+
+Each release entry follows this structure:
+
+```markdown
+## [X.Y.Z] - YYYY-MM-DD
+
+### Added
+- New features
+
+### Changed
+- Changes to existing functionality
+
+### Deprecated
+- Features that will be removed in future versions
+
+### Removed
+- Features that were removed
+
+### Fixed
+- Bug fixes
+
+### Security
+- Vulnerability fixes
+```
+
+## Categories
+
+Use these categories in the order listed:
+
+| Category | When to use |
+|---|---|
+| **Added** | New features, new CLI commands, new checks, new output formats |
+| **Changed** | Changes to existing behavior, scoring weight adjustments, API changes |
+| **Deprecated** | Features marked for future removal |
+| **Removed** | Features that have been removed |
+| **Fixed** | Bug fixes, corrected scoring, fixed edge cases |
+| **Security** | Vulnerability patches, dependency updates for security |
+
+## Guidelines
+
+- Write entries from the user's perspective, not the developer's
+- Start each entry with a verb (Add, Fix, Change, Remove, etc.)
+- Reference issue numbers where applicable
+- Group related changes into a single entry when they form a logical unit
+- Keep entries concise but descriptive enough to understand without reading the code
+- Unreleased changes go under an `## [Unreleased]` section at the top
+
+## Version Links
+
+At the bottom of `CHANGELOG.md`, include comparison links:
+
+```markdown
+[Unreleased]: https://github.com/hanselhansel/context-cli/compare/v3.0.0...HEAD
+[3.0.0]: https://github.com/hanselhansel/context-cli/compare/v2.0.0...v3.0.0
+[2.0.0]: https://github.com/hanselhansel/context-cli/releases/tag/v2.0.0
+```
+
+## Versioning
+
+Context CLI follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html):
+
+- **Major** (X.0.0) -- breaking changes to CLI interface, scoring model changes, API changes
+- **Minor** (0.Y.0) -- new features, new commands, new checks
+- **Patch** (0.0.Z) -- bug fixes, documentation updates, dependency updates