Arc Implementation Plan Progress

Create two synchronized in-flight artifacts under the project docs/ directory:

• docs/<ID>-IMPLEMENTATION_PLAN.md or a clearly named variant
• docs/<ID>-progress.txt with checkbox execution tracking mapped to the plan phases

The leading word is synchronized: every progress item must trace to the plan, and every plan phase must have progress coverage.

File naming & lifecycle

• Unique prefix. Prefix both artifacts with a unique work-item identifier so concurrent plans never collide in docs/ — use the tracked work-item ID when one exists (e.g. docs/W-000025-IMPLEMENTATION_PLAN.md, docs/W-000025-progress.txt), otherwise a short kebab-case feature slug (e.g. docs/rate-limit-IMPLEMENTATION_PLAN.md). The plan and its progress file MUST share the same prefix.
• Archive on completion. When the work is complete (all progress items [x] / the shipping PR merges), move both files into docs/archive/ so docs/ only ever shows in-flight plans. Use scripts/archive_plan.sh <plan-file> <progress-file>, or git mv them manually. Record this move as the final progress step.