feat(search): Tavily + Brave behind a search-provider port; Google CSE retires

Search port + model slots wave, chunk A (owner design, 2026-09-20):
- core/search.ts: SearchHit + SearchProvider port + SearchStatus —
  pure, browser-safe; adapters arrive as injected dependencies.
- server/search.ts: Tavily + Brave adapters (wire shapes VERIFIED
  against docs.tavily.com's OpenAPI + Brave's dashboard docs — Tavily
  auth is the Bearer header, not the legacy body api_key; Brave is
  X-Subscription-Token) + engine resolution per owner decisions: a key
  ALONE activates its engine (no engine choice needed), explicit
  agent.search.engine pins the preference (both keys: Tavily first),
  a pinned engine with a missing key is honestly unavailable — no
  silent fallback, no default engine.
- Curated keys catalog: GOOGLE_SEARCH_API_KEY/GOOGLE_CSE_ID retire
  (CSE closed to new customers, dies Jan 2027 — stored values become
  preserved unknown vars); TAVILY_API_KEY (tvly- shape) + BRAVE_API_KEY
  (length guard) join. UI keys-section TOOL_IDS now derives from the
  catalog — cannot drift.
- GET /api/search: {available, engine, reason} — the honest status for
  the settings gray-out; researcher-disable semantics ride the same
  signal (Teacher stream rewires the tool onto the port).
- agent.search settings field (schema + sanitizer + strict parser);
  web.ts's WebSearchResult is now the core SearchHit (re-exported).

tests/settings-body.test.ts

@@ -64,6 +64,18 @@ describe("parseAgentSettingsBody", () => {
 		// cross-validation — the parser only sees shapes.
 	});
 
+	it("search: engine pin is tavily|brave; anything else is a 400 (search wave, 2026-09-20)", () => {
+		const ok = parseAgentSettingsBody({ search: { engine: "tavily" } });
+		assert.ok(ok.ok);
+		assert.deepEqual(ok.settings.search, { engine: "tavily" });
+		const clear = parseAgentSettingsBody({ search: {} });
+		assert.ok(clear.ok);
+		assert.equal(clear.settings.search, undefined, "absent engine = back to auto-detect");
+		assert.equal(parseAgentSettingsBody({ search: { engine: "google" } }).ok, false);
+		assert.equal(parseAgentSettingsBody({ search: "tavily" }).ok, false, "search must be an object");
+		assert.equal(parseAgentSettingsBody({ search: { engine: 1 } }).ok, false);
+	});
+
 	it("an empty {} agent section parses to {} (replace-whole clear)", () => {
 		const r = parseAgentSettingsBody({});
 		assert.ok(r.ok);