Synology Act Runner Split PR Body

Title

chore: split Synology act_runner start and re-registration scripts

Added tools/re_register_act_runner_synology.sh for explicit host-mode re-registration.
Added tools/start_act_runner_synology.sh for boot-time daemon start only.
Kept tools/setup_act_runner.sh as the bootstrap path, but made the re-registration flow explicit and repeatable.
Switched the runner registration labels to self-hosted:host,snapshot-admin-host:host so the job runs in host mode instead of Docker job containers and can be targeted by a dedicated deployment label.
Updated docs/SYNOLOGY_SNAPSHOT_ADMIN_POC.md and docs/ROADMAP_WBS.md so the operator flow and WBS notes match the new runner split.
The snapshot_admin.yml workflow is split into push smoke validation and manual full validation, which reduces routine CI cost while preserving the full web smoke path on demand.

python tools/validate_snapshot_admin_workflow_v1.py
python -c "import yaml, pathlib; yaml.safe_load(pathlib.Path('.gitea/workflows/snapshot_admin.yml').read_text(encoding='utf-8'))"
git diff -- .gitea/workflows/snapshot_admin.yml tools/setup_act_runner.sh docs/SYNOLOGY_SNAPSHOT_ADMIN_POC.md docs/ROADMAP_WBS.md