fix(sandbox): support write_file append in FsBridge (#650)

- Add appendMode bool param to FsBridge.WriteFile (>> vs >)
- Pass appendMode through sandbox path in WriteFileTool
- EditTool always uses overwrite mode (false)

Closes #650

internal/tools/filesystem_write.go

@@ -13,14 +13,14 @@ import (
 
 // WriteFileTool writes content to a file, optionally through a sandbox container.
 type WriteFileTool struct {
-	workspace        string
-	restrict         bool
-	deniedPrefixes   []string // path prefixes to deny access to (e.g. .goclaw)
-	sandboxMgr       sandbox.Manager
-	contextFileIntc  *ContextFileInterceptor // nil = no virtual FS routing
-	memIntc          *MemoryInterceptor      // nil = no memory routing
-	permStore     store.ConfigPermissionStore // nil = no group write restriction
-	workspaceIntc *WorkspaceInterceptor      // nil = no team workspace validation
+	workspace       string
+	restrict        bool
+	deniedPrefixes  []string // path prefixes to deny access to (e.g. .goclaw)
+	sandboxMgr      sandbox.Manager
+	contextFileIntc *ContextFileInterceptor     // nil = no virtual FS routing
+	memIntc         *MemoryInterceptor          // nil = no memory routing
+	permStore       store.ConfigPermissionStore // nil = no group write restriction
+	workspaceIntc   *WorkspaceInterceptor       // nil = no team workspace validation
 }
 
 // DenyPaths adds path prefixes that write_file must reject.
@@ -148,7 +148,7 @@ func (t *WriteFileTool) Execute(ctx context.Context, args map[string]any) *Resul
 	// Sandbox routing (sandboxKey from ctx — thread-safe)
 	sandboxKey := ToolSandboxKeyFromCtx(ctx)
 	if t.sandboxMgr != nil && sandboxKey != "" {
-		return t.executeInSandbox(ctx, path, content, sandboxKey, deliver)
+		return t.executeInSandbox(ctx, path, content, sandboxKey, deliver, appendMode)
 	}
 
 	// Host execution — use per-user workspace from context if available
@@ -222,7 +222,7 @@ func (t *WriteFileTool) Execute(ctx context.Context, args map[string]any) *Resul
 	return result
 }
 
-func (t *WriteFileTool) executeInSandbox(ctx context.Context, path, content, sandboxKey string, deliver bool) *Result {
+func (t *WriteFileTool) executeInSandbox(ctx context.Context, path, content, sandboxKey string, deliver, appendMode bool) *Result {
 	bridge, err := t.getFsBridge(ctx, sandboxKey)
 	if err != nil {
 		return ErrorResult(fmt.Sprintf("sandbox error: %v", err))
@@ -234,11 +234,19 @@ func (t *WriteFileTool) executeInSandbox(ctx context.Context, path, content, san
 	}
 	containerPath := ResolveSandboxPath(path, containerCwd)
 
-	if err := bridge.WriteFile(ctx, containerPath, content); err != nil {
-		return ErrorResult(fmt.Sprintf("failed to write file: %v", err) + MaybeFsBridgeHint(err))
+	if err := bridge.WriteFile(ctx, containerPath, content, appendMode); err != nil {
+		verb := "write"
+		if appendMode {
+			verb = "append to"
+		}
+		return ErrorResult(fmt.Sprintf("failed to %s file: %v", verb, err) + MaybeFsBridgeHint(err))
 	}
 
-	msg := fmt.Sprintf("File written: %s (%d bytes)", path, len(content))
+	verb := "written"
+	if appendMode {
+		verb = "appended"
+	}
+	msg := fmt.Sprintf("File %s: %s (%d bytes)", verb, path, len(content))
 	if deliver {
 		msg += ". File will be automatically delivered to the user — do NOT send it again via message tool."
 	}