docs(roadmap): add #409 — init artifact schema: redundant parallel arrays; skip_reason absent

[code-yeongyu]

Pinpoint #409

init --output-format json emits two parallel artifact representations (structured artifacts[{name,status}] + flat created[]/skipped[]/updated[] string arrays) that carry identical state with no additional info between them. Additionally artifacts[].status:"skipped" carries no reason field — automation cannot distinguish already_exists from permission_denied, dry_run, or conflict.

Evidence:

# Fresh dir:
{"artifacts":[{"name":".claw/","status":"created"},…], "created":[".claw/",…], "skipped":[], "updated":[]}

# Existing dir (re-run):
{"artifacts":[{"name":".claw/","status":"skipped"},…], "created":[], "skipped":[".claw/",…], "updated":[]}
# No reason field on any artifact entry; prose only in message: "skipped (already exists)"

ROADMAP-only PR. No source/test changes.