feat: customize model provider requests

Model connections can now save bounded JSON request headers and top-level body fields. Direct requests and Continue support both; local OpenCode and DeepSeek Harness use only native header support, while the managed remote OpenCode gateway applies both.

Runtime, protocol, and authentication fields keep precedence. Custom values are ordinary connection settings, not credential storage. This migrates Runtime settings from v20 to v21 and updates the DeepSeek control and managed Runtime ACP capabilities.

Release note: 模型连接详情新增自定义请求头和请求体；直连与 Continue 支持两者，本地 OpenCode 和 DeepSeek Harness 仅应用原生支持的请求头，受管远程 OpenCode 支持两者，界面会明确提示生效范围。

FEATURES.md

@@ -103,7 +103,7 @@ otherwise.
   the Agent tool-dispatch boundary, while Execute has all permissions of the
   selected SSH account. The Agent owns accepted Prompts,
   provider/tool rounds, Runtime processes, a stable model ledger, and a bounded
-  semantic transcript over a private Unix socket and ACP v4. Work continues on
+  semantic transcript over a private Unix socket and ACP v5. Work continues on
   the Host after Desktop exit, network loss, or local-process termination.
   Reconnection attaches only to the original controller, binding, and
   operation, then atomically merges provenance into the original conversation
@@ -213,6 +213,14 @@ otherwise.
   a bounded real text or image-generation request and validates the generated
   result instead of testing HTTP reachability alone, so it may incur a small
   provider usage charge.
+- [x] **Custom model request fields**: Each LLM connection accepts bounded
+  JSON objects for additional request headers and top-level body fields.
+  Direct requests and Continue support both; local OpenCode and DeepSeek
+  Harness use only their natively supported headers, while the managed SSH
+  OpenCode model gateway supports both. Runtime, protocol, and authentication
+  fields take precedence, and these ordinary connection settings must not be
+  used to store API keys or other secrets. See
+  [model request customization](./docs/features/model-connections/README.md).
 - [x] **Context usage and automatic compaction**: Direct models update usage
   from each successful provider call. Images and tool rounds use the same
   accounting, with estimation only when the provider omits usage. The UI
@@ -509,7 +517,7 @@ otherwise.
 - [ ] **GoodBuddy Team Hub** (planned): An optional service for organizations,
   RBAC, project sharing, remote Agents, policy distribution, and tenant audit.
 - [ ] **SSH Host and remote execution-space release acceptance**: Host CRUD,
-  Host Key, encrypted credentials, Project UI, Workspace, OpenCode ACP v4,
+  Host Key, encrypted credentials, Project UI, Workspace, OpenCode ACP v5,
   Agent-owned Prompt/gateway/transcript, read-only Ask, full-account Execute,
   cancellation, exact detached-Agent reconnection, and release-only
   dual-architecture resource verification are wired. Current Linux x64 source