Normalize DevTools workspace directory handling

[coisa]

Related Issue

Closes #274

Motivation / Context

• --working-dir now selects the project root, but generated artifacts and caches still needed one central policy instead of each command assuming .dev-tools independently.
• Consumer and global-binary usage need a way to move generated reports, coverage, metrics, and caches without changing source/resource lookup semantics.
• Nested orchestrators such as reports and standards should carry the same workspace defaults without requiring duplicated command-specific paths.

Changes

• Adds the standalone global --workspace-dir/-w option and wires it to FAST_FORWARD_WORKSPACE_DIR before command defaults are resolved.
• Teaches ManagedWorkspace to resolve output/cache roots from the configured workspace while keeping .dev-tools as the default.
• Keeps explicit command paths authoritative, including --target, --coverage, --metrics, and --cache-dir.
• Extends tooling source-scan exclusions and PhpMetrics defaults so a custom relative workspace is not analyzed as project source.
• Updates README, command docs, usage docs, and tooling defaults to distinguish project root from generated artifact workspace.
• Adds tests for default workspace behavior, custom relative/absolute workspaces, source-scan exclusions, metrics excludes, and the DevTools global option.

Verification

• composer dev-tools
• composer dev-tools:fix
• composer dev-tools docs
• Focused command(s): composer dev-tools tests -- --filter='(ManagedWorkspaceTest|WorkingProjectPathResolverTest|DevToolsTest|MetricsCommandTest)'
• Manual verification: php bin/dev-tools.php --workspace-dir=.artifacts reports --help --no-ansi | rg '\.artifacts|workspace-dir'
• Manual verification: FAST_FORWARD_WORKSPACE_DIR=.artifacts php bin/dev-tools.php metrics --help --no-ansi | rg '\.artifacts|exclude'
• Manual verification: confirmed help checks did not create .artifacts

Documentation / Generated Output

• README updated
• docs/ updated
• Generated or synchronized output reviewed

Changelog

• Added a notable CHANGELOG.md entry

Reviewer Notes

• --working-dir continues to define where DevTools runs; --workspace-dir only changes generated artifact/cache defaults.
• Composer plugin usage can use FAST_FORWARD_WORKSPACE_DIR for the same workspace policy because Composer cannot receive DevTools-specific global application options.
• Existing explicit output/cache options continue to win over workspace defaults.

[github-actions]

🚀 Preview is available for this pull request.

• Docs: https://php-fast-forward.github.io/dev-tools/previews/pr-275/
• Coverage: https://php-fast-forward.github.io/dev-tools/previews/pr-275/coverage/
• Metrics: https://php-fast-forward.github.io/dev-tools/previews/pr-275/metrics/