per-session models: every session owns its model + thinking level from birth

The reported bug (2026-09-28): every chat ran glm-5.3-flash regardless of
any pick, while composer and settings UIs agreed with each other and lied
about the run. Root cause: a fossil default-settings record (settings.json
chat section, frozen pre-slots-wave) was stamped verbatim into every new
chat at birth by the new-session flow, and resolveConversationSpec prefers
the per-chat record over the strong slot forever.

Owner redesign — models are per session, always; the strong slot is the
new-session default:

- POST /api/chats stamps the newborn verbatim from the strong slot
  (model + thinking level); no client settings PUT on create anymore
- the composer control is dual-bound: in-session it reads/writes the
  session's own record (GET /api/chats/:id, PUT .../settings); on the
  draft surfaces it binds the strong slot (picking there = changing the
  default)
- thinking levels follow the identical mechanics (confirmed by owner)
- the zombie default-settings system dies whole: useDefaultSettings,
  /api/settings/defaults, src/store/default-settings.ts, the chat
  section of UserSettings (sanitizer strips fossil copies), the legacy
  default-settings.json fold, and DEFAULT_SETTINGS_FILE
- catalogDefaultsFor collapses to the strong slot alone
- thinking display for a legacy unstamped session resolves exactly like
  the engine (session ?? strong ?? model ladder)

Tests: new chat-birth-stamp suite pins the stamp, the unconfigured boot,
the unset level, and the copy-not-live-view regression; defaults pins
rewritten strong-only; legacy-migration suite deleted.

tests/settings-resolution.test.ts

@@ -24,7 +24,6 @@ describe("resolveSettings", () => {
 	it("null record (first boot): non-model knobs land on factory defaults; every model view is EMPTY", () => {
 		const eff = resolveSettings(null);
 		assert.deepEqual(eff.general, { autoUpdate: true });
-		assert.deepEqual(eff.chat, {}); // BYOK: no default model, ever
 		assert.deepEqual(eff.agent.tools, {});
 		assert.deepEqual(eff.agent.prompts, {});
 		assert.deepEqual(eff.agent.search, {});
@@ -51,7 +50,6 @@ describe("resolveSettings", () => {
 		const record: UserSettings = {
 			version: 1,
 			general: { autoUpdate: false },
-			chat: { thinkingLevel: "high" },
 			agent: {
 				models: { researcher: { model: { provider: "openrouter", id: "qwen/qwen3.8-flash" } } },
 				tools: { research: false },
@@ -63,7 +61,6 @@ describe("resolveSettings", () => {
 		};
 		const eff = resolveSettings(record);
 		assert.equal(eff.general.autoUpdate, false);
-		assert.deepEqual(eff.chat, { thinkingLevel: "high" });
 		assert.deepEqual(eff.agent.models.researcher, {
 			provider: "openrouter",
 			id: "qwen/qwen3.8-flash",