feat: add before_message_write plugin hook

Synchronous hook that lets plugins inspect and optionally block messages
before they are written to the session JSONL file. Primary use case is
private mode... when enabled, the plugin returns { block: true } and the
message never gets persisted.

The hook runs on the hot path (synchronous, like tool_result_persist).
Handlers execute sequentially in priority order. If any handler returns
{ block: true }, the write is skipped immediately. Handlers can also
return a modified message to write instead of the original.

Changes:
- src/plugins/types.ts: add hook name, event/result types, handler map entry
- src/plugins/hooks.ts: add runBeforeMessageWrite() following tool_result_persist pattern
- src/agents/session-tool-result-guard.ts: invoke hook before every originalAppend() call
- src/agents/session-tool-result-guard-wrapper.ts: wire hook runner to the guard

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/plugins/types.ts

@@ -309,6 +309,7 @@ export type PluginHookName =
   | "before_tool_call"
   | "after_tool_call"
   | "tool_result_persist"
+  | "before_message_write"
   | "session_start"
   | "session_end"
   | "gateway_start"
@@ -493,6 +494,18 @@ export type PluginHookToolResultPersistResult = {
   message?: AgentMessage;
 };
 
+// before_message_write hook
+export type PluginHookBeforeMessageWriteEvent = {
+  message: AgentMessage;
+  sessionKey?: string;
+  agentId?: string;
+};
+
+export type PluginHookBeforeMessageWriteResult = {
+  block?: boolean;      // If true, message is NOT written to JSONL
+  message?: AgentMessage; // Optional: modified message to write instead
+};
+
 // Session context
 export type PluginHookSessionContext = {
   agentId?: string;
@@ -575,6 +588,10 @@ export type PluginHookHandlerMap = {
     event: PluginHookToolResultPersistEvent,
     ctx: PluginHookToolResultPersistContext,
   ) => PluginHookToolResultPersistResult | void;
+  before_message_write: (
+    event: PluginHookBeforeMessageWriteEvent,
+    ctx: { agentId?: string; sessionKey?: string },
+  ) => PluginHookBeforeMessageWriteResult | void;
   session_start: (
     event: PluginHookSessionStartEvent,
     ctx: PluginHookSessionContext,