fix(workspace): add artifact-only cleanup plans

[chubes4]

Summary

• Add a plan-first worktree artifact cleanup command for profile-derived build outputs.
• Keep artifact cleanup separate from whole-worktree removal so build directories can be reviewed and removed without deleting worktrees.

Changes

• Adds datamachine/workspace-worktree-cleanup-artifacts and wp datamachine workspace worktree cleanup-artifacts.
• Requires dry-run JSON before deletion and revalidates worktree metadata plus exact artifact paths when applying a plan.
• Protects dirty/unpushed worktrees by default and always protects active Studio plugin/theme symlink targets.
• Adds artifact-specific summary stats separate from worktree cleanup/removal stats.
• Adds pure-PHP smoke coverage for planning, apply-plan deletion, plan mismatch rejection, and safety protections.

Closes #145

Tests

• php -l inc/Workspace/Workspace.php
• php -l inc/Cli/Commands/WorkspaceCommand.php
• php -l inc/Abilities/WorkspaceAbilities.php
• php -l tests/smoke-worktree-cleanup-artifacts.php
• php -l tests/smoke-worktree-cleanup-cli.php
• php tests/smoke-worktree-cleanup-artifacts.php
• php tests/smoke-worktree-cleanup-cli.php
• php tests/smoke-worktree-cleanup.php
• git diff --check
• homeboy audit data-machine-code --path /Users/chubes/Developer/data-machine-code@feat-worktree-artifact-cleanup-profiles --changed-since origin/main

Known environment/pre-existing blockers:

• homeboy lint data-machine-code --path /Users/chubes/Developer/data-machine-code@feat-worktree-artifact-cleanup-profiles --changed-since origin/main reports existing PHPCS/PHPStan findings in touched files and smoke harnesses.
• homeboy test data-machine-code --path /Users/chubes/Developer/data-machine-code@feat-worktree-artifact-cleanup-profiles fails during WordPress Playground/Data Machine activation before DMC tests with SQLSTATE[42S02]: Base table or view not found: 1146 Table 'wptests_options' doesn't exist.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Implemented the artifact-cleanup command/ability and smoke tests; Chris reviews and validates before merge.