V2: decide session context / guidance / instructions terminology #35057

Split out of #35017 / #35021. The vocabulary around model-visible ambient context is inconsistent and needs one deliberate decision before the event/API names freeze for beta:

Current overlapping terms

• SystemContext — the source/reconcile algebra (packages/core/src/system-context)
• SessionContextEntry — API-provided keyed entries (api/<key> sources, session/context-entry.ts)
• SessionContextCheckpoint — the durable per-source Applied baseline (session/context-checkpoint.ts)
• session.next.context.updated — the event narrating drift as System updates (projected as SessionMessage.System)
• "guidance" — the name V2: add structured guidance/context update summaries #35017 proposed for the event (session.guidance.updated), already used by skill/reference/mcp source modules (skill/guidance.ts etc.)
• "instructions" — candidate rename for session context entries (also currently SessionInstructions = AGENTS.md loading, and instruction-context.ts = ambient instructions source)

To decide

1. One name for the model-facing ambient layer: context vs guidance vs instructions — and which existing modules adopt it.
2. The event name: the V2: event audit tracker #35021 audit renames session.next.context.updated → session.context.updated (mechanical prefix drop only). If a different final name wins, it lands with this issue's follow-through.
3. Whether the projected message type stays "system".
4. Whether SessionContextEntry / its /api/session/:id/context surface rename accordingly.

Deferred alongside: #35017's structured changes payload (additive later; parked head-start branch system-context in worktree opencode-system-context).