Show specific release

[obstschale]

Summary

• output unreleased changes as Markdown from the show command
• add optional version argument to print one exact release section from CHANGELOG.md
• return a non-zero error when the requested release is missing

Closes #84

Tests

• vendor/bin/phpunit tests/Feature/Commands/ShowTest.php --colors=always

[obstschale]

Code review

1. general (medium) - ShowCommand.php: generateContent() duplicates the markdown generation logic from ReleaseCommand; extracting a shared formatter would avoid future inconsistencies.
2. general (low) - ShowCommand.php: parsing unreleased LogEntry files is duplicated from ReleaseCommand; consider moving that mapping into ChangesDirectory.

Checks: code review inspected origin/master...HEAD. No automated checks were run as part of this review.

[obstschale]

Updated the PR with a refactor that extracts shared changelog markdown generation into ChangelogMarkdownGenerator and wires both show and release through it.

Tests:

• vendor/bin/phpunit tests/Feature/Commands/ShowTest.php tests/Feature/Commands/ReleaseTest.php --colors=always