Add environment and writable path diagnostics

[josiahking]

Summary

• add a configurable Doctor check for required environment variables
• report environment-variable presence without exposing values
• add a configurable Doctor check for writable local filesystem paths
• keep writable-path inspection read-only with no automatic filesystem changes
• document both checks as programmatic project diagnostics
• update the accepted Core source inventory

Behavior

Environment variable diagnostics treat only a missing process variable as absent; explicitly defined empty-string values remain present. Required names are validated, duplicate definitions are rejected, lookup order is deterministic, and environment values are never included in findings or remediation.

Writable-path diagnostics inspect explicitly supplied local paths, preserve their supplied order, reject invalid or duplicate definitions, and report non-writable paths without creating directories or changing permissions.

These checks are not automatically added to the package-owned shell Doctor. Existing shell behavior remains unchanged.

Validation

• Core test suite passes
• full quality suite passes on PHP 8.4
• static analysis passes with no errors
• coding-style validation passes
• architecture and documentation tests pass
• release package validation passes
• prerelease consumer validation passes
• deterministic package split validation passes
• Composer manifest, security, supply-chain, and license checks pass