wave 2 (platform stream): settings storage & plumbing foundation (P2)

- src/core/settings.ts: versioned DELTA record schema + sanitizer (unknown keys preserved)
  — general/chat/agent{models,tools,prompts}/appearance/graph sections
- src/core/settings-resolution.ts: pure resolveSettings (factory defaults → user delta),
  extraction/graph-context channel predicates
- src/store/settings.ts + prompt-overrides.ts: SettingsStore/PromptOverrideStore adapters
  wired through stores.ts (effectiveSettings() fresh per call); lazy migration of
  default-settings.json into record.chat
- app/api/settings/** routes (record, section bodies, prompt overrides per segment)
- chat-handler: graph gates at trigger call-sites + toolToggles injection seam
- config.ts: SETTINGS_FILE + PROMPTS_OVERRIDE_DIR paths

587/587 tests; typecheck/lint/build clean

app/api/settings/prompts/route.ts

@@ -0,0 +1,29 @@
+/**
+ * GET /api/settings/prompts — the system-prompt segment view
+ * (settings-storage wave, 2026-09-19): the effective prompt segments for
+ * BOTH agents, derived from SOURCE at runtime (src/agent/prompts — the
+ * list adapts when the code ships/renames/removes fragments), each with
+ * its source text, override text (if a file exists), and effective switch
+ * state (settings record's agent.prompts map AND file presence).
+ *
+ *   GET → 200 {prober: SegmentInfo[], teacher: SegmentInfo[]}
+ *   SegmentInfo = {path, sourceText, overrideText, enabled, shared}
+ *
+ * The shared base appears in BOTH lists under the SAME path
+ * ("system-prompt.md") — the prompt-editor's "what's constant between the
+ * two agents" is discoverable by path equality. Writes: the switches go
+ * through PUT /api/settings/agent; the TEXT goes through
+ * PUT/DELETE /api/settings/prompts/[...segment].
+ */
+
+import { effectivePromptSegments, readSettingsRecord } from "@/src/server";
+
+export const dynamic = "force-dynamic";
+
+export async function GET() {
+	const switches = readSettingsRecord()?.agent?.prompts ?? {};
+	return Response.json({
+		prober: effectivePromptSegments("prober", switches),
+		teacher: effectivePromptSegments("teacher", switches),
+	});
+}