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

src/server/index.ts

@@ -12,6 +12,20 @@ export * from "./jobs.ts";
 export * from "./stores.ts";
 export { readRenderSettings, writeRenderSettings } from "../store/render-settings.ts";
 export { readDefaultSettings, writeDefaultSettings } from "../store/default-settings.ts";
+// The settings record + prompt overrides (settings-storage wave, 2026-09-19).
+// The *Record aliases keep the app/api/settings routes explicit about which
+// seam they touch (the raw record, not the chat-section convenience view).
+export {
+	readSettings as readSettingsRecord,
+	writeSettings as writeSettingsRecord,
+	settingsStore,
+} from "../store/settings.ts";
+export {
+	promptOverrides,
+	effectivePromptSegments,
+	allSegmentPaths,
+	isValidSegmentPath,
+} from "../store/prompt-overrides.ts";
 export * from "./settings-body.ts";
 export * from "./turn-liveness.ts";
 export * from "./observability.ts";