Warn on base branches checked out in worktrees

[chubes4]

Summary

• Flag non-primary worktrees checked out on base/protected branch names so gh pr merge --delete-branch cleanup failures are visible before merges.
• Surface the warning in worktree list results, hygiene summaries, and human CLI output.
• Add smoke coverage for the base-branch worktree warning path.

Closes #404.

Testing

• php -l inc/Workspace/WorkspaceWorktreeLifecycle.php
• php -l inc/Workspace/WorkspaceHygieneReport.php
• php -l inc/Cli/Commands/WorkspaceCommand.php
• php tests/smoke-worktree-list-scaling.php
• homeboy test --path /Users/chubes/Developer/data-machine-code@fix-worktree-base-branch-warnings --extension wordpress

Notes

• homeboy lint --path /Users/chubes/Developer/data-machine-code@fix-worktree-base-branch-warnings --extension wordpress still fails on existing unrelated lint debt outside this change.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Filing the issue, implementing the worktree warning surfaces, and running focused smoke/activation checks. Chris identified the confusing gh pr merge --delete-branch failure mode from the Homeboy Extensions merge.