Schedule safe workspace cleanup by default

[chubes4]

Summary

• add an enabled-by-default hourly workspace safe-cleanup maintenance task
• compose the existing WorkspaceSafeCleanupOrchestrator
• bound runs by item count, passes, cycles, and stage budget
• keep force and unpushed-discard modes disabled
• expose execution through existing task, schedule, health, and cleanup evidence surfaces

Testing

• php tests/workspace-safe-cleanup-schedule.php
• php tests/workspace-safe-cleanup-orchestrator.php
• adjacent cleanup status, capacity, and evidence tests
• PHP syntax checks
• git diff --check origin/main...HEAD

Closes #255.

AI assistance

OpenAI GPT-5.6 Sol using OpenCode drafted the implementation and focused tests through a general coding subagent. The resulting diff was rebased and independently re-run against the targeted tests before publication. Chris Huber remains responsible for the change.