fix(graph-context): honest wording for the unconfigured-embedder failure

Wave-6 review (Teacher stream): the injection runner's null-embedder
branch failed with 'graph.embeddingModel is null', but the resolver
collapses BOTH an unset model AND a set model whose provider key
vanished into null — a key race mid-session would have reported a
false 'null' setting. The message now names both causes honestly.
Pinned by a new failure-mode test (null resolver → honest error, never
a phantom query).

Review verdicts (no changes needed): the session-deletion guard sits at
every chat-merge boundary (per batch, fresh lookup, rides the existing
AbortError path — nothing merged, nothing checkpointed, registry
cleaned in the finally); persistRunUsage's deleted-chat skip composes
with appendSourceUsage's vanished-source no-op so no usage record
dangles; notes sources have no deletion surface so the notes path needs
no guard. The embedding tail's non-fatal contract holds (failure →
recordEmbedFailure → no stage, extractor/auditor usage still persists
through the catch).

src/agent/subagents/graph-context.ts

@@ -145,9 +145,11 @@ export async function runGraphContextJob(deps: GraphContextDeps, jobId: string):
 	// BYOK (wave 6): fresh resolution per run; null = honestly off (the
 	// dispatch gate already skips unconfigured sessions — this only fires
 	// on a config change racing a respawn, and the error says exactly why).
+	// The resolver returns null for BOTH an unset model and a set model
+	// whose key vanished, so the message names both, never a false "null".
 	const embedder = deps.embedder();
 	if (!embedder || embedder.model === null) {
-		return fail("no embedding model configured — injection skipped (graph.embeddingModel is null)");
+		return fail("no embedding model available — injection skipped (graph.embeddingModel unset, or its provider key is missing)");
 	}
 	const topic = topicOf(chat.messages);
 	if (topic === null) return fail("no injectable topic (session past its start)");