Add scheduled workspace hygiene reports

[chubes4]

Problem

Workspace bloat is invisible until macOS complains or a command times out. DMC should proactively surface workspace health because it owns the runtime/workspace bridge.

Proposed solution

Add a scheduled workspace hygiene report, likely as a Data Machine system task registered by DMC.

The task should summarize:

• Total workspace size.
• Free disk space.
• Worktree count.
• Top repos by worktree size.
• Biggest cleanup candidates.
• Dirty/unpushed protected worktrees.
• Missing metadata / external worktree skips.
• Suggested cleanup command.

Delivery options

• CLI command for on-demand report.
• Recurring Data Machine system task for daily/weekly checks.
• Optional agent memory/log entry or Discord notification depending on site configuration.

Acceptance criteria

• There is an on-demand command that prints the hygiene report.
• There is a scheduled task hook/handler that can run the same report.
• The report does not require destructive actions.
• The report has JSON output for automation.
• The report highlights emergency thresholds, e.g. free disk below 20 GiB.

Example summary

Workspace hygiene report
- Free disk: 12.4 GiB (warning)
- Workspace size: 512.7 GiB
- Worktrees: 423
- Top repo: homeboy, 188 worktrees, 481.2 GiB
- Cleanup candidates: 91 clean merged/closed worktrees, estimated 274.3 GiB
- Protected: 12 dirty, 4 unpushed, 3 active symlink targets

Suggested next command:
wp datamachine-code workspace worktree cleanup --dry-run --sort=size