Harden Decodex stall progress accounting

[yvette-carlisle]

Summary

• split protocol liveness from meaningful work progress for run activity markers
• keep account, rate-limit, phase-goal, passive status, warning, token-usage, and similar control traffic from refreshing last_progress_at
• surface progress_diagnostic = "protocol_only_activity" when model execution has fresh protocol traffic but stale work progress
• update operator docs/specs and add regression coverage for account/rate-limit plus phase-goal protocol events

Validation

• cargo test -p decodex run_protocol_non_work_events_do_not_refresh_progress_marker --all-features -- --test-threads=1
• cargo test -p decodex operator_status_snapshot_diagnoses_protocol_only_model_execution --all-features -- --test-threads=1
• cargo make fmt
• cargo make lint-fix
• cargo make test

Fixes XY-918