fix(#492): manifest effort.agent_overrides and effort.default now fall back correctly

Steps 2 and 4 of resolveEffortInternal now include an else branch that
consults CANONICAL_CONFIG_DEFAULTS.effort when effortCfg is null, mirroring
the existing Step 3 manifest-fallback pattern for routing_tier_defaults.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/bug-492-effort-manifest-fallback.test.cjs

@@ -0,0 +1,50 @@
+'use strict';
+
+process.env.GSD_TEST_MODE = "1";
+
+const { describe, test, beforeEach, afterEach } = require("node:test");
+const assert = require("node:assert/strict");
+const path = require("path");
+const { createTempProject, cleanup } = require("./helpers.cjs");
+const { resolveEffortInternal, CONFIG_DEFAULTS } = require("../get-shit-done/bin/lib/core.cjs");
+const { CONFIG_DEFAULTS: CANONICAL_CONFIG_DEFAULTS } = require("../get-shit-done/bin/lib/configuration.cjs");
+
+describe("#492 manifest effort fallback", () => {
+  let tmpDir;
+  beforeEach(() => { tmpDir = createTempProject(); });
+  afterEach(() => { cleanup(tmpDir); });
+
+  test("routing_tier_defaults manifest fallback still works", () => {
+    assert.strictEqual(resolveEffortInternal(tmpDir, "gsd-planner"), "xhigh");
+  });
+
+  test("manifest effort.agent_overrides wins over routing_tier_defaults when no project config", () => {
+    const original = CANONICAL_CONFIG_DEFAULTS.effort.agent_overrides;
+    try {
+      CANONICAL_CONFIG_DEFAULTS.effort.agent_overrides = { "gsd-planner": "max" };
+      assert.strictEqual(resolveEffortInternal(tmpDir, "gsd-planner"), "max");
+    } finally {
+      CANONICAL_CONFIG_DEFAULTS.effort.agent_overrides = original;
+    }
+  });
+
+  test("manifest effort.default consulted for unknown agent with no project config", () => {
+    const original = CANONICAL_CONFIG_DEFAULTS.effort.default;
+    try {
+      CANONICAL_CONFIG_DEFAULTS.effort.default = "max";
+      assert.strictEqual(resolveEffortInternal(tmpDir, "fictional-agent-xyz-492"), "max");
+    } finally {
+      CANONICAL_CONFIG_DEFAULTS.effort.default = original;
+    }
+  });
+
+  test("manifest agent_overrides takes precedence over manifest routing_tier_defaults", () => {
+    const originalAgentOverrides = CANONICAL_CONFIG_DEFAULTS.effort.agent_overrides;
+    try {
+      CANONICAL_CONFIG_DEFAULTS.effort.agent_overrides = { "gsd-planner": "minimal" };
+      assert.strictEqual(resolveEffortInternal(tmpDir, "gsd-planner"), "minimal");
+    } finally {
+      CANONICAL_CONFIG_DEFAULTS.effort.agent_overrides = originalAgentOverrides;
+    }
+  });
+});