docs: use standard Keep a Changelog categories

Replace non-standard "Improved" section with "Changed" in CHANGELOG.md
and add explicit Keep a Changelog category requirement to CLAUDE.md.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sjsyrek

CHANGELOG.md

@@ -7,25 +7,23 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
-### Improved
+### Changed
 - "No target language specified" error now shows clearer suggestions: `--to <lang>` example and `deepl init` instead of raw JSON config syntax
 - Write service error now says `--style` and `--tone` instead of leaking internal API field name `writing_style`
 - `deepl config set` validation errors now include the field name and valid values (e.g., "Invalid formality for defaults.formality. Valid values: default, more, less, prefer_more, prefer_less")
 - `deepl config set` boolean validation now shows which field expected a boolean
 - Document translation timeout message simplified: removed internal poll-count details
 - Batch translation API errors now show a user-friendly retry message instead of internal count details
+- Updated GitHub Actions (`actions/checkout`, `actions/setup-node`) from v4 to v6 across all CI workflows
+- Updated `@types/node` from v20 to v25
+- Updated inquirer from 12 to 13
+- Updated better-sqlite3 from 11 to 12
 
 ### Fixed
 - Double space in "Unsupported file type" suggestion message for file translation
 - Config save error no longer shows redundant `Error: ` prefix or internal `.tmp` file path
 - Unreachable `Invalid hook type` error in git-hooks service now includes valid types (matching the reachable validation)
 
-### Changed
-- Updated GitHub Actions (`actions/checkout`, `actions/setup-node`) from v4 to v6 across all CI workflows
-- Updated `@types/node` from v20 to v25
-- Updated inquirer from 12 to 13
-- Updated better-sqlite3 from 11 to 12
-
 ## [1.0.0] - 2026-02-17
 
 ### Added

CLAUDE.md

@@ -68,7 +68,7 @@ Use **Semantic Versioning** with **Conventional Commits**:
 
 ### On Every Change
 
-1. Add entries under **Unreleased** in `CHANGELOG.md` (Added / Changed / Fixed / Deprecated / Removed / Security)
+1. Add entries under **Unreleased** in `CHANGELOG.md` using **only** the standard [Keep a Changelog](https://keepachangelog.com) categories: **Added**, **Changed**, **Deprecated**, **Removed**, **Fixed**, **Security**. Do not invent custom categories (e.g., "Improved").
 2. Determine MAJOR/MINOR/PATCH from change scope
 
 ### When Cutting a Release