feat: share built-in browser across runtimes

The built-in browser was limited to direct-model tools, and its Agent switch also blocked manual Browser workbar actions. It is now a request-scoped GoodBuddy capability assignable to direct models, managed OpenCode, and Continue, while manual browser controls remain available when Agent access is off.

Existing browser enablement migrates to capability settings version 6. DeepSeek Harness remains unsupported.

Release note: 内置浏览器现在可分配给直连模型、OpenCode 和 Continue；关闭 Agent 浏览器能力不会影响用户在浏览器工作栏中的手动操作。

src/shared/capability-contracts.ts

@@ -38,7 +38,8 @@ export type CapabilityAssignments = z.infer<
 export const builtinMcpServerIdSchema = z.enum([
   'knowledge-base',
   'magic-notes',
-  'goodbuddy-config'
+  'goodbuddy-config',
+  'builtin-browser'
 ])
 export type BuiltinMcpServerId = z.infer<
   typeof builtinMcpServerIdSchema
@@ -369,7 +370,7 @@ export const capabilitySnapshotSchema = z
     skills: z.array(skillSummarySchema).max(256),
     builtinMcpServers: z
       .array(builtinMcpServerStateSummarySchema)
-      .max(3)
+      .max(4)
       .optional(),
     mcpServers: z.array(mcpServerSummarySchema).max(64),
     webSearch: webSearchCapabilitySchema.optional(),