docs: update README.md & rename .env vars

[dominnya]

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[Copilot]

Pull Request Overview

This PR renames environment variables from GRATIO_* to OPENAPI_* to better reflect their purpose and improve clarity. The package version is bumped to 3.0.0 to indicate this breaking change.

• Renamed all environment variable references from GRATIO_* prefix to OPENAPI_* prefix
• Updated documentation (README.md and CHANGELOG.md) to reflect the new environment variable names
• Bumped version to 3.0.0 in package.json

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/commands/gen/types.ts	Updated environment variable references from GRATIO_TYPES_* to OPENAPI_TYPES_*
src/commands/gen/endpoints.ts	Updated environment variable references from GRATIO_ENDPOINTS_* to OPENAPI_ENDPOINTS_*
package.json	Bumped version from 2.0.0 to 3.0.0
README.md	Added documentation for .env configuration and updated CLI help text with new environment variable names
CHANGELOG.md	Added version 3.0.0 entry documenting the environment variable renaming

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[Copilot]

The section heading '## Структура пакета' appears outside of a version entry, which breaks the Keep a Changelog format. This section should either be part of the [3.0.0] release entry (as a subsection with ### heading) or removed if it duplicates information from earlier versions.