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/[section]/route.ts

@@ -0,0 +1,86 @@
+/**
+ * PUT /api/settings/[section] — section-scoped record writes
+ * (settings-storage wave, 2026-09-19). One PUT replaces ONE section
+ * replace-whole (absent fields = absent overrides — the house settings
+ * discipline, same as the per-chat and defaults routes); sections are
+ * independent, so partial writes can never clobber each other.
+ *
+ *   section ∈ general | chat | agent | appearance | graph (else 404).
+ *   Body: the complete desired section object. Parse + validate strictly
+ *   (server/settings-body.ts): a bad shape/type is a 400 carrying the
+ *   supported list where one exists, never a silent clamp. The chat
+ *   section additionally passes the catalog gate (validateChatSettings —
+ *   unknown provider/model or unsupported thinking level = 400), same as
+ *   the defaults route. The agent section additionally validates prompt
+ *   switches: an ON switch must reference a CURRENT source segment with an
+ *   existing override file (an OFF switch may reference anything — users
+ *   cleaning up orphans must not be blocked).
+ *
+ * The static siblings (defaults, prompts) take precedence over this
+ * dynamic segment — Next resolves static routes first, so "prompts" never
+ * lands here.
+ */
+
+import type { AgentSettings, ChatSettings } from "@/src/core";
+import {
+	SETTINGS_SECTIONS,
+	allSegmentPaths,
+	promptOverrides,
+	readSettingsRecord,
+	validateChatSettings,
+	writeSettingsRecord,
+	type SettingsBodyResult,
+} from "@/src/server";
+
+export const dynamic = "force-dynamic";
+
+/** One parser, result type erased to the common envelope. */
+type AnySectionParser = (body: unknown) => SettingsBodyResult<unknown>;
+
+export async function PUT(request: Request, ctx: { params: Promise<{ section: string }> }) {
+	const { section } = await ctx.params;
+	const parser = (SETTINGS_SECTIONS as Record<string, AnySectionParser>)[section];
+	if (!parser) {
+		return Response.json(
+			{ error: `unknown settings section "${section}"`, supported: Object.keys(SETTINGS_SECTIONS) },
+			{ status: 404 },
+		);
+	}
+	const body = await request.json().catch(() => null);
+	const parsed = parser(body);
+	if (!parsed.ok) {
+		return Response.json({ error: parsed.error }, { status: 400 });
+	}
+
+	// Section-specific cross-validation (against the catalog / the prompt
+	// store) — the parsers only see shapes.
+	if (section === "chat") {
+		const verdict = validateChatSettings(parsed.settings as ChatSettings);
+		if (!verdict.ok) {
+			return Response.json({ error: verdict.error, supported: verdict.supported }, { status: 400 });
+		}
+	}
+	if (section === "agent") {
+		const agent = parsed.settings as AgentSettings;
+		const segments = new Set(allSegmentPaths());
+		for (const [path, on] of Object.entries(agent.prompts ?? {})) {
+			if (on && (!segments.has(path) || promptOverrides.readOverride(path) === null)) {
+				return Response.json(
+					{
+						error: `prompt switch for "${path}" cannot be on: ${
+							segments.has(path) ? "no override file exists" : "not a current source segment"
+						}`,
+					},
+					{ status: 400 },
+				);
+			}
+		}
+	}
+
+	// Read-modify-write ONE section of the record; everything else rides
+	// along untouched (other sections, unknown keys — sanitize preserves).
+	const record = readSettingsRecord() ?? { version: 1 };
+	const next = { ...record, [section]: parsed.settings };
+	writeSettingsRecord(next);
+	return Response.json({ settings: next });
+}