Generalizes #32663 (@ehz0ah). The slash-skill scaffolding pollution
affected every auto-syncing memory provider — mem0, hindsight, retaindb,
byterover, honcho, supermemory all store/embed the raw user turn, so a
/skill invocation poisoned their stores with the full skill body, not just
openviking.
- Lift the contributor's parser into agent/skill_commands.py as the canonical
extract_user_instruction_from_skill_message(), co-located with the message
builders so the markers can't drift.
- Strip once in MemoryManager.{prefetch_all,queue_prefetch_all,sync_all} —
fixes the whole provider fan-out, bare /skill turns are skipped entirely.
- OpenViking's _derive_openviking_user_text() now delegates to the shared
helper as defense-in-depth (no duplicated marker literals).
- Marker-drift regression now asserts against the canonical skill_commands
constants; add manager-level coverage proving every provider gets clean text.
