fix: update bundled DeepSeek Harness to rc.8

The desktop runtime still pinned the rc.6 Harness family. It now uses the coherent rc.8 package set, adapts the revised image-attachment limits and error contract, and derives release checks from the lockfile. The managed SSH Agent package remains unchanged.

Release note: 内置 DeepSeek Harness 已升级至 0.1.0-rc.8，并适配新版图片附件限制；托管 SSH Agent 包不受影响。

src/main/agent/runtime-discovery.test.ts

@@ -192,7 +192,7 @@ describe('runtime discovery', () => {
       binaryPath: '',
       bundledPath: bundledScript,
       bundledValidation: 'canonical-file',
-      bundledVersion: '0.1.0-rc.6',
+      bundledVersion: '0.1.0-rc.8',
       binaryNames: [],
       label: 'GoodBuddy DeepSeek Harness Host'
     })
@@ -200,7 +200,7 @@ describe('runtime discovery', () => {
     expect(detection).toMatchObject({
       available: true,
       path: await realpath(bundledScript),
-      version: '0.1.0-rc.6',
+      version: '0.1.0-rc.8',
       source: 'bundled'
     })
     expect(detection.detail).toContain('内置')