Wave 0: shared contracts + extension points

- src/core/contracts.ts: plan tree / graph / ledger / job protocol / artifact
  registry types — the single source of truth for all workstreams
- tools: per-request factory createTeacherTools({chatId}) composing
  questionTools + planTools (stub) + jobTools (stub); quiz gains nodeId
- prompt composition: system-prompt.md + sorted src/agent/prompts/*.md
- ChatRecord.tree + saveTree; pending gate scoped to quiz/ask (+findPendingJobCall)
- chat.tsx: artifact renderer extension point + ChatActionsContext.syncFromServer

src/agent/prompts/README.md

@@ -0,0 +1,13 @@
+# Prompt fragments
+
+The teacher's system prompt is composed at agent creation:
+
+1. `src/agent/system-prompt.md` — the base (owned by the orchestrator; do not edit from a worker).
+2. Every `*.md` file in this directory, in **sorted filename order**, joined with `---` separators.
+
+Rules for adding a fragment:
+
+- Name it with a numeric order prefix: `10-plan-tree.md`, `20-jobs.md`, `30-phases.md`.
+- One fragment per workstream — your brief tells you which filename you own.
+- Document YOUR tools/behaviors exactly; reference tool input field names verbatim (they must match the zod schemas).
+- Keep it tight: fragments are always in context. No restating the base prompt.