Feature/improve cloud schedule

[ErraticO]

Summary

• aigear-scheduler CLI: Expanded from 1 command (--create) to 8 full lifecycle commands — --create, --update, --delete, --status, --list, --run, --pause, --resume
• Scheduler class (scheduler.py): Refactored with _make_scheduler / _build_messages helpers; added module-level functions for all 8 operations; added docstrings to all methods
• GCP infra CLI (gcp_cli.py): Added --delete command
• CloudKMS: Added delete(), describe_enabled_key_version(), create_key_version(), and renamed create_key_version → enable_primary_key_version
• Artifacts / IAM / PreVMImage: Added delete() method to each
• VM image baking: Parallelized CPU and GPU baking steps
• Cost estimation doc: Added docs/cost-estimation.md with cost breakdown and advantage highlights
• Docs: Updated docs/cli-reference.md and README.md to reflect all scheduler commands

CLI usage (aigear-scheduler)

aigear-scheduler --create  --version <v> --step_names <s1,s2> --env staging
aigear-scheduler --update  --version <v> --step_names <s1,s2> --env production
aigear-scheduler --delete  --version <v>
aigear-scheduler --status  --version <v>
aigear-scheduler --list    --version <v>
aigear-scheduler --run     --version <v>
aigear-scheduler --pause   --version <v>
aigear-scheduler --resume  --version <v>