Deduplicate the maintenance_* and ps_* command families

[justin808]

Summary

Two command families duplicate their structure file-by-file, so any interface or behavior change must be repeated in every sibling:

• Maintenance family — maintenance_on.rb (29 lines), maintenance_off.rb (29), maintenance_set_page.rb (34): identical NAME/OPTIONS scaffolding delegating to one MaintenanceMode method each.
• Process-control family — ps_start.rb, ps_stop.rb, ps_restart.rb, ps_wait.rb: each re-implements single-vs-all-workloads resolution and the wait/poll pattern (step + retry + max attempts).

Proposed fix

• Introduce a shared intermediate base per family (e.g. MaintenanceCommand < Base providing memoized maintenance_mode + shared OPTIONS; a workload-iteration/wait helper for the ps_* family), keeping each command file as a thin declaration.
• Constraint to verify first: command discovery works by regex-scanning source files for class X < ...Base (Command::Base.all_commands) — intermediate base classes must still be picked up (the pattern intends to match subclasses of any *Base; add a spec proving discovery still finds all commands, and that the new intermediate classes are NOT registered as commands).
• One PR per family.

Acceptance criteria

• Wait/poll logic exists once for the ps_* family; maintenance scaffolding exists once
• cpflow --help output and commands.md (rake check_command_docs) unchanged
• Spec asserting command discovery still registers every command and no intermediate bases
• Behavior-preserving: fast suite green

Priority / size / dependencies

P3. Size: M. Soft-blocked by #384 (sequence after the Command::Base extraction to avoid repeated churn in the same files; can proceed independently if #384 stalls).

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Origin: automated codebase health review, 2026-07-02, against 7c96df9.