WEB-1194: Add Composer vcs repositories and install docs

[miguelcolmenares]

Summary

Adds a Composer vcs repository and install documentation so this package and its SilverAssist development dependency resolve from GitHub instead of Packagist.org. Phase 0 of WEB-1194 (making the SilverAssist package repositories private without breaking Composer). The token support for private releases follows separately in 1.4.0.

Changes

• composer.json: vcs repository for silverassist/coding-standards (development dependency).
• README: new "Installing via Composer (private repository)" section (vcs entry for consumers, token setup, lock refresh, troubleshooting).
• CHANGELOG: 1.3.2 entry.
• create-release.yml: the generated release notes point at the README install section instead of Packagist.
• .github/copilot-instructions.md: no longer says the package is distributed through Packagist.

No runtime code changes.

Not in this PR

The Packagist badges in the README, .packagist.yml and the .gitattributes comment stay until the packages are removed from Packagist. examples/integration-guide.php is updated with the 1.4.0 documentation pass.

Verification

• composer validate --strict passes (the local composer.lock is git-ignored and unrelated).
• composer install from a clean directory with the new vcs entry resolves silverassist/coding-standards from GitHub.
• Error messages in the troubleshooting table were reproduced with Composer 2.8.10.
• The edited workflow file parses as valid YAML; the release notes heredoc keeps the existing escaped backticks.

Release

Merging is followed by tag v1.3.2.