feat: implement garbage-collect for all providers

[frostebite]

Summary

Implements proper garbage collection for all main providers, fixes --garbageMaxAge wiring, and ensures --dry-run works correctly.

• AWS: Fixed garbageMaxAge (was converted to a boolean, now passes actual hours). Fixed buggy isOlderThan1day which used getDay() (returns 0-6) instead of computing actual age. Added dry-run output logging and returns a summary of actions taken.
• K8s: Full implementation — cleans up completed/failed jobs, orphaned pods, build-credentials-* secrets, and service-account-* service accounts older than maxAge. Respects --dry-run and --filter.
• Docker: Full implementation — removes stopped unity containers, prunes dangling images (or all unity images with --fullCache), and removes orchestrator volumes. Respects --dry-run and --garbageMaxAge.
• CLI serve command: Wires --garbageMaxAge from CLI options into the garbage-collect dispatch (was previously ignored, always passed 0).

Addresses feedback from game-ci/documentation#541 (review comment about safety/confirmation).

Test plan

• game-ci garbage-collect --provider-strategy aws --dry-run shows what would be deleted without deleting
• game-ci garbage-collect --provider-strategy aws --garbageMaxAge 48 only targets resources >48h old
• game-ci garbage-collect --provider-strategy k8s --dry-run lists stale jobs/pods/secrets
• game-ci garbage-collect --provider-strategy local-docker --dry-run lists stale containers/images
• All existing tests still pass (848 passing)

🤖 Generated with Claude Code

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