FN-6745: add xhigh thinking level support

Adds the xhigh reasoning option across settings, task flows, APIs, and model adapter tests.

- Extend the shared thinking-level enum, validation paths, generated resource types, and settings documentation for `xhigh`.
- Surface `xhigh` in task, agent, and global model selectors with localized labels.
- Cover task route handling, task form rendering, and Pi adapter mapping behavior with regression tests.
- Add a patch changeset for the published Fusion package.

Files changed:
 .changeset/fn-6745-xhigh-thinking-level.md         |   5 +
 docs/settings-reference.md                         |   2 +-
 .../core/src/__tests__/thinking-levels.test.ts     |   9 ++
 packages/core/src/types.ts                         |  10 +-
 packages/dashboard/app/api/legacy.ts               |   6 +-
 .../dashboard/app/components/AgentDetailView.tsx   |   2 +-
 .../dashboard/app/components/ModelSelectorTab.tsx  |   1 +
 .../dashboard/app/components/NewAgentDialog.tsx    |   3 +-
 packages/dashboard/app/components/NewTaskModal.tsx |   2 +-
 .../dashboard/app/components/TaskDetailModal.tsx   |   2 +-
 packages/dashboard/app/components/TaskForm.tsx     |   2 +
 .../app/components/__tests__/TaskForm.test.tsx     |  14 +++
 .../settings/sections/GlobalModelsSection.tsx      |   1 +
 .../src/__tests__/routes-tasks-ops.test.ts         |   8 +-
 .../dashboard/src/__tests__/routes-tasks.test.ts   |   8 +-
 packages/dashboard/src/agent-generation.ts         |   4 +-
 packages/dashboard/src/agent-onboarding.ts         |   4 +-
 .../src/routes/register-task-workflow-routes.ts    |   5 +-
 packages/engine/src/__tests__/pi.test.ts           |  25 ++++
 packages/i18n/locales/en/app.json                  |   3 +
 packages/i18n/locales/es/app.json                  |   3 +
 packages/i18n/locales/fr/app.json                  |   3 +
 packages/i18n/locales/ko/app.json                  |   3 +
 packages/i18n/locales/zh-CN/app.json               |   3 +
 packages/i18n/locales/zh-TW/app.json               |   3 +
 packages/i18n/src/resources.d.ts                   | 126 ++++++++++++++++++++-
 26 files changed, 229 insertions(+), 28 deletions(-)

Fusion-Task-Id: FN-6745

Fusion-Task-Lineage: 779e1517-607b-482b-83a1-0f2bb94ae736

packages/dashboard/app/api/legacy.ts

@@ -3411,7 +3411,7 @@ export interface AgentOnboardingSummary {
   name: string;
   role: AgentCapability | "custom";
   instructionsText: string;
-  thinkingLevel: "off" | "minimal" | "low" | "medium" | "high";
+  thinkingLevel: "off" | "minimal" | "low" | "medium" | "high" | "xhigh";
   maxTurns: number;
   title?: string;
   icon?: string;
@@ -3444,7 +3444,7 @@ export interface ExistingAgentOnboardingConfig {
   reportsTo?: string;
   skills?: string[];
   model?: string;
-  thinkingLevel?: "off" | "minimal" | "low" | "medium" | "high";
+  thinkingLevel?: "off" | "minimal" | "low" | "medium" | "high" | "xhigh";
   maxTurns?: number;
   runtimeHint?: string;
   heartbeatIntervalMs?: number;
@@ -6352,7 +6352,7 @@ export interface AgentGenerationSpec {
   /** Detailed system prompt in markdown */
   systemPrompt: string;
   /** Suggested thinking level */
-  thinkingLevel: "off" | "minimal" | "low" | "medium" | "high";
+  thinkingLevel: "off" | "minimal" | "low" | "medium" | "high" | "xhigh";
   /** Suggested max turns (1-500) */
   maxTurns: number;
 }