Graph redesign: true propositions, follows/supports, auditor-owned identity (vision §1 minimal implementation)

The minimal implementation of the 2026-09-02 graph vision (docs/vision.md §1):

- contracts/graph.ts: every node is a TRUE proposition (statement is both
  rendering label and identity key; required description; no kind, no name).
  Edges are follows (derivable) / supports (grounds without entailing) with
  message-granular provenance points {chatId, messageId}. Events reduce to
  correct/miss/dont-know plus observation (understanding notes); no taught
  events, no misconception kind.
- extraction.ts: extractor is TREE-BLIND (transcript only, no plan tree) and
  free-form; auditor sees findings + the current graph, owns faithfulness and
  identity (link-or-create verdict per proposition, recorded as linkTo);
  transcriptView tags every line with its message id (provenance anchors).
- core/graph.ts merger: applies verdicts deterministically — linkTo appends
  a link line (phrasings join aliases, provenance unions; reads merge by id),
  absent creates; NO string matching anywhere. Quiz events rebuilt from tool
  parts with attribution-only nodeId resolution; understanding notes become
  observation events (obs:<sourceId>); mapping lines are the verdict audit
  trail and make re-merges idempotent.
- deriveNodeStates: transitional — latest quiz event wins (known/missed/
  dont-know, else candidate); taught unreachable until the state-model
  ideation lands.
- graph-context.ts + 50-graph-context.md: follows chains, hotspot grouping,
  state vocabulary updated for the new event kinds.
- graph-view.tsx: node label is the statement; detail shows statement +
  description.
- tests: merger verdicts (link, no-string-match, cross-chat assertions),
  tree-blind pipeline end-to-end, follows vocabulary in context tests.
  276/276 green.

src/agent/prompts/50-graph-context.md

@@ -1,20 +1,19 @@
 # The learner's knowledge graph
 
-After these instruction fragments you may see a fenced block titled **LEARNER KNOWLEDGE GRAPH (auto-generated)**. It is the accumulated record of what this learner has actually demonstrated, across every past session — rebuilt fresh from quiz outcomes and plan trees, every turn. Each line is a proposition in its current state (in brackets), and `a → b` under **Requires** means *a* must be understood before *b*.
+After these instruction fragments you may see a fenced block titled **LEARNER KNOWLEDGE GRAPH (auto-generated)**. It is the accumulated record of what this learner has actually demonstrated, across every past session — rebuilt fresh from quiz outcomes, every turn. Each line is a proposition in its current state (in brackets), and `a → b` under **Follows** means *b is derivable from a*: understanding a makes understanding b achievable.
 
 The states, exactly:
 
 - **[missed]** — answered wrong. Treat as a live misconception site until a check proves otherwise.
 - **[dont-know]** — the learner declined to guess. Unexplored, not broken.
 - **[known]** — answered correctly. Foundation you can build on and refer to by name.
-- **[taught]** — a lesson covered it. Check that it stuck before stacking anything heavy on it.
-- **[candidate]** — asserted into a plan but never probed. Assume nothing.
+- **[candidate]** — present in the graph but never probed. Assume nothing.
 
 This block is what makes probing precise instead of generic — use it:
 
-- **Probe the frontier, not the interior.** [known] and [taught] nodes near the session's topic are the learner's foundation: reference them by name and build on them — never re-teach what the graph says he holds. The sharpest probe questions live one step past the known strip, at the [missed] and [dont-know] nodes.
+- **Probe the frontier, not the interior.** [known] nodes near the session's topic are the learner's foundation: reference them by name and build on them — never re-teach what the graph says he holds. The sharpest probe questions live one step past the known strip, at the [missed] and [dont-know] nodes.
 - **[missed] nodes are your first stops.** A wrong answer is a misconception until shown otherwise. Open the session by re-probing those — not the same question again, but the subtlety underneath the miss.
-- **Read the Requires chains as scaffolding.** They show what the learner's knowledge rests on. If a prerequisite under the session's topic is [missed] or [dont-know], that is where teaching starts — not at the topic itself.
+- **Read the Follows chains as scaffolding.** They show what the learner's knowledge rests on. If a premise under the session's topic is [missed] or [dont-know], that is where teaching starts — not at the topic itself.
 
 When the block is **absent**, the learner is new (or nothing is recorded yet): probe from scratch, make no references to past sessions, and let today's outcomes plant the first nodes.